1. create your character in the character editor and save them
2. In the Experiment GameMode (enter from main menu) go to Custom Potions tab and select the character, press the + button
3. Drag the potion onto a subject (drag it into quick select screen first), it will transform them into your custom character
4. place a bed in the experiment area (Press the bed button and place it somewhere). Then drag the characters onto it.
5. SEGGS! :P
PS. press RMB on the bed for the sex menu ;) pick positions etc.