I always think about this!
What I would do depends on the time period, although in all scenarios I would let the generals focus on the details and I will focus on the bigger picture, along with minimising all atrocities and repealing all anti semitic laws and bringing Jews to work in industry and the military to increase my bargaining position and strength. Total war, the mobilisation of all resources for the war effort, will be declared as early as possible. The big projects, such as the Maus, Ratte, Lowe, the ME 262 jet fighter, the V-2, V-1, V-3, and other rocket projects, as well as other Wunderwaffe, as these were too new, would have had many technological problems (like in real life), and whilst the US could invest in such new technologies, Germany couldn’t when fighting a multiple front war, with fewer resources, and most of the fighting taking place on the Eastern Front, where the completed Wunderwaffe were never used. Finally, the E series of tanks would be built as early as possible. I will only build the E-75 Standardpanzer, as powerful as a Tiger Two, but much cheaper and less complicated to build and maintain, and the E-25 Jagdpanzer. I will change Enigma codes regularly to make it harder to defeat the U-boats, which I will do with all the scenarios here. Another thing I will do in all scenarios is to create an anti aircraft defence like the British did to defend against bombing attacks.
1939: I’ll let the Polish campaign as it is, and don’t invade Scandinavia. This way, Chamberlain would stay in power during the Battle Of France, and when Churchill rises, he would be in them middle of a crisis than at the beginning, and coupled with the encirclement and destruction of the BEF, sue for peace with Britain. After that, don’t declare war on the US, and go all in on the Soviet Union. I would also recruit the Soviet population to fight for Germany. Many people in the Soviet Union hated Stalin and were more than happy to overthrow the Soviet government. Two examples in real life were the defection of a Soviet regiment and a request from officials from Smolensk after the capture of the city, to create a Russian Liberation Army. I will say that they should join Germany, I will have them treated as nicely as possible, create self governing regions, create local forces to eliminate a lot of partisan activity. As for the German navy, I would finish the Seydlitz and Graf Zepplin, and use the German surface fleet to engage in a massive battle with the Royal Navy to lower British morale. I would also let the navy develop their own branch of land based aircraft for more coordination. I would also do this for the 1941 scenario.
Beginning of 1941: Go straight up to Franco and the leaders of Turkey and tell them that they have to join the Axis or be invaded. Don’t invade the Soviet Union yet, and strike a good deal with Vichy France-I will probably mediate so that Italy and Spain get what they want from French North Africa, and give Vichy France as compensation large amounts of Sub-Saharan Africa that Italy doesn’t want. With a full push in the Mediterranean, the British would not help Greece out, capturing Tripoli, but allowing the Germans to move into Tunisia and push the British back into Libya, hopefully crossing the Egyptian border by the end of 1941. Whilst all of this is happening, an airborne invasion of Malta, with Crete not garrisoned, will be launched, and after the capture of Gibraltar through Spain, will ensure the safe supply of resources to Africa. More German troops (possibly an entire Panzer Army), along with Vichy forces, would raise Italian morale. More oil will be allocated for the Italian navy, raising morale. By 1942, the Axis would have pushed through into the Middle East, and a peace treaty should be signed as soon as possible with Britain-even if the Middle East is safe in British hands, a southern front against the Soviets can still be launched through Turkey. I will order the construction of the E-series tanks as fast as possible, but only two types: the Jagdpanzer E-25 tank destroyer, and the E-75 Standard Panzer, as powerful as the Tiger Two, but much cheaper, simpler, and easier to produce. I will not declare war on the US, and by 1942 or 1943, launch Operation Barborossa, and despite more Soviet T-34s and KV-1s and 2s, and more preparations and alertness, the power of the E-75, additional Turkish, Vichy French, Italian, and Spanish forces, bombardment by the Italian navy in the Black Sea, German forces not needed from the Mediterranean, and the opening of a southern front through Turkey would make victory much more likely. I would also recruit the Soviet population, as shown above. By the end of 1944, the Axis should have captured Moscow, and Ukraine, Belarus, Poland, Lithuania, Latvia, and Estonia should be independent countries, and all the nations of Eastern Europe should be united by an alliance, headed by Germany. The Vichy French navy is a powerful force and can wreak havoc in the Mediterranean, which can be used with the Italian navy later in the war in the Black Sea.
Beginning of 1942: I would invade Malta before Rommel’s invasion with paratrooper forces I will try to rebuild as fast as possible (not sacrificing quality, of course). Germany should launch an offensive towards the Caucasus, and not invade Stalingrad. The original plan for Operation Case Blue should be followed as shown below:
https://en.wikipedia.org/wiki/Case_Blue
Hopefully Rommel will succeed in North Africa, which he can then turn his attention against the American Operation Torch landings. Again, I will try to convince Spain, Vichy France, and Turkey to join the war, to make the Middle East more accessible and link up with the Axis in the Caucausus. Again, I will recruit the Soviet population, and despite lots of atrocities being committed, there still can be hope. Hopefully, with a more successful 1942 summer offensive, a peace deal can be achieved. The Graf Zepplin should be completed, as should the Seydlitz. I would do the Channel Dash as in real life, and use all large German ships to be stationed in Norway (including repairing the Gneisenau), and use them to attack Allied convoys. As many ships as possible should be committed for a single operation at once for maximum effect, and to be able to destroy more Allied naval units to gain a better peace treaty.
Late 1942: I will not invade Kursk, instead using those forces to defend on the Eastern Front, recruit the Soviet population (which will still be willing despite large amounts of atrocities occurring), and order the Sixth Army to break out during Operation Winter Storm, hopefully escaping. As Army Group A would not be able to capture anything significant anyway, it should be used to help breaking the encirclement. As many Axis forces in Tunisia should be evacuated as possible, hopefully delaying the Italian surrender. The plan for the German navy would be the same as in early 1942. As the Allies would probably win the Battle Of The Atlantic by now, I would abandon submarine production by summer 1943.
Early 1944: I would propose a fantastic peace deal to the Western Allies, where I would give up the entire Kriegsmarine, give the “10 Countries”: Luxembourg, France, Belgium, the Netherlands, Greece, Albania, Denmark, Yugoslavia, Italy, and Norway. Poland will still be under control by Germany, due to the connection with the rest of Eastern Europe. The puppet states of Croatia and the Italian Social Republic will be dissolved, and the Western Allies can influence all of these countries to their liking. If that fails, I would also give the Allies “The Extra Offer Of 5 Countries”: Bulgaria, Estonia, Czechoslovakia, Lithuania, and Latvia. These countries can be influenced on the same terms as the first 10, and the puppet state of Slovakia will be dissolved. My only demand with all of these countries is that they should remain neutral in my war with the Soviet Union. Once the peace treaty has been signed, I would then move all of Germany’s mechanised forces to the Belarus, until I have 30 mechanised divisions there, ready to counter Operation Bagration. Hopefully the attack is stopped, and then I can move my mechanised forces southwards to counter Soviet offensives there as well. Hopefully, with my superior tanks, as powerful as the Tiger Two, but available in more numbers and having less mechanical breakdowns, and the entire Luftwaffe being able to concentrate on the Eastern Front, and resources used from naval production, I can defeat the Soviets and sign a peace treaty.