First go to manage worlds and select "Manage Households" menu. Choose your household and "Transfer Sims Between Households." It will take you to another menu (Household Split and Merge), from there click the "Create New Household" button at the top right side and move over the individual Sim. Then click the check. Now you've moved out the sim from the original household, but he still needs his own home. You mentioned that you figured out how to do this part already.
Still in Manage Households, click on the Individual Sim who should now be appearing in he Played Households list on his own. A small menu should pop up with him and his household information. At the bottom, select the button that says "Move Household into Lot." Click that and move him into any home you want. Now anytime you load the save, you can choose which household to play and switch back and forth between the family and the individual.
...That's pretty long and confusing, sorry. I'll try to edit it and add screenshots later to make it more clear.
