If you've got your fry tank set up and the water is all good, try putting some plants in and somewhere for the fry to hide.
Then transfer the mother molly into the fry tank and let her give birth there, then remove her when she's done - this is much easier and safer than trying to catch and transfer the fry from another tank, and they are born in the water conditions that they are going to live in, so there less chance of stress.
Don't overdo it with the salt, a little will do.
In fact, a touch of salt and a slight rise in temperature will induce your mother to give birth.
When the fry are born feed them baby brine shrimp. One of the main problems with feeding flake food straight away is that it's very easy to overdo it in the first week and pollute the tank. Live food at least stays that way until the babies have had a chance to eat it!
Good Luck!
P.S. Livebearer fry are tougher than you think - I've moved 20 - 30 Platy fry by netting them in the last few weeks and they are all doing well!