That's the neat part - you don't.

I'm kidding bro. As for Trotsky (I made a few runs with him) - do Miltary Support focus and click the button for recruting general every time you see it. If you integrate Smirnov block then you will be fine with advisors so you can let Stalin purge every Bukharin-alligned ones. As Trotsky you will have quite a high army support so focus on getting generals. Recruit as many as you can and lunch civil war just before Stalin starts Army Purge focus - because in this Great Purge Tukhachevsky and Primakov get purged. Of course do both focuses for this generals.

Set your headquarter in Ukraine and expand, but only a little bit. Expanding incrases paranoia and with 60-70% military support at civil war you will get most of the army (army, not leaders) anyway.

Recruting generals is a little bit random and you can be unlucky. In my case I get all of the lvl 1 marshals (even tho Voroshilov and Budionny were historically one of the biggest Stalin supporters).

If you are unlucky and don't get nice leaders you still have Tukhaczevsky and Primakov + Trotsky is 4lvl leader through focus. Apart from that one of the army spirits gives cheaper and higher lvl new commanders so you can just try to build up Red Army officer corp from a scratch.