Well it will be fairly easy at this time of year as there will not be / should be no babies
1. Find out where they are getting in - there maybe more than one hole.
2. If more than one hole block the ones least visable.
3. Give them food (outside) and see if you can work out how many are up there!
4. Keep feeding and get them used to you being there as you watch them go in and out.
5. When you know this block the hole when you see the last one come out!
6. put water and a little bit of food in the loft for about a week - just to be on the safe side

7. Be patient if the squirrels aren't used to you being around this may take a few days

8. It's something I've done for neighbours etc on numerous occasions so it does work!
Feel free to drop me a pm or open message if you get into difficulties!