You get yourself organised both financially and mentally.
Financially, get yourself into a position where you can earn enough to support yourself, that may include changing jobs so that you have child care compatible hours, or doing some extra training. Have your own savings accounts and make sure any debt in your name is paid off.
If your H is an alcoholic then you wouldn't want him responsible for childcare so start documenting what he does childcare wise and what you do.
If your DC are getting close to the age where their wishes will be taken into account and they wouldn't want to stay with him, maybe hold out for a smidgen extra until they are old enough. Caveat to that - don't do it if you think making that sort of a decision would be difficult for them.
Maybe document his drinking habits, alcohol receipts would be sufficient if they're now coming out of his account, if you can get to them.
Get all financial documents together. Find out his pensions, bank accounts, earnings. If he is self employed get accurate documentation regarding his earnings so he can't reduce his earnings to avoid paying child support.
Shore up your support network. Start socialising more, while keeping your DC safe, obviously. Keep your friends away from him, stop socialising with his friends. You may as well start what is going to happen later on.
And be a warned that once you start doing this, your H is likely going to notice you pulling away and make things harder for you. So do the big things first in case you are forced into moving out earlier than you planned.