1. Choose Auto-Generated fleet.
2. Finances.
Max crew training and transports and build shipyards. Set tech to zero, it's expensive and tech won't help you at all in this timeframe.
3. Ship design.
Basically we need to optimize for two things: the blockade calculation and the battle auto-resolver.
For blockade, heavier ships are worth more per ton. So pick the heaviest ship you can, probably a BB. Then, max op range.
The battle auto-resolver is less important since the blockade can win the war on its own. Still, you want to make sure that you win auto-resolves as much as possible, which gives you VP directly, and more importantly, reduces your casualties, making it easier to maintain the blockade.
Make sure you have:
A lot of single torpedo launchers (including underwater), which are highly valued by the auto-resolver. You don't need torpedo size, ammo, or multiple tubes per launcher, just a lot of launchers.
High numerical armor. It doesn't have to be balanced, you can just slap on 50" belt and call it a day.
You can invest some weight into speed if you have cheap engines, but it's not essential.
Guns, accuracy, bulkheads, balance, detonation chance etc. aren't worth worrying about. Just pick the cheapest components so you can afford a lot of tonnage.
Example design:
4. Playing the game.
Just auto-resolve all battles while building as many max-size BBs as funds/crew allow. (Note that at current the auto-crew won't un-mothball ships, so dropping below zero crew pool means you'll need to manually add crew.) Eventually you'll blockade the enemy into a revolution and win the war.
Britain has an easier time at blockades due to their larger economy, but I've gotten this to work with Germany as well.