This is how I did it. It seems so daunting at first, but once you get it all straight it is very liberating to be in control of your own finances.
First of all, get your own bank account set up. Make sure you are receiving any lone parent/low income discounts and benefits on your council tax/water bills etc and that tax credits are aware of your situation.
Sit down and work out what each of you has to pay to run your own separate household. Also how much you each have coming in (including child benefit, tax credits but not any payments from him to you just yet).
Figure out how much the CSA recommended amount is based on how many DCs you have, how often he has them and what he earns. This is the very minimum he should pay you.
If you were renting instead of mortgaged you would get help with housing benefit, but this won't apply if you are planning to stay put, so if your ex wants to enable you and your DCs to remain in the family home he needs to account for this.
He will also presumably have pensions and greater earning potential for the future, so when dividing up assets for divorce this should be reflected in your settlement (I have agreed to take 100% of he equity in return for no extra spousal maintenance on top of CSA and no claim on his pensions etc).
I work pt and have been able to stay in the house, I intend to keep his name on the mortgage (he won't be able to buy anywhere else anyway, and the bank won't accept me on my own as I don't earn enough, so he will have to accept that the mortgage remains technically 50% his responsibility, even though I will pay it, in order to give his DCs security) I then need to get his name taken off the deeds, so that he will be on the mortgage, but without having a stake in the proceeds.