Great question — answering as the UphillTech team:
You paint on only one sheet at a time, removing it and placing it on a table, easel, or other surface of your choice. You can use a hard or stiff backboard if needed. It does take a long time for oils to dry.
Full answer by Patricia Mcgood: https://crafts.stackexchange.com/a/8062 (CC BY-SA 4.0).
Need it done for your business? UphillTech.