Use the palette knife to make individual piles of the leftover oil paint mixes on the freezer paper or in the plastic container. Try not to smear the paint. Freezer paper is easier to use since it takes less space and can be disposed of later. The choice is yours.
Note the color mixes with a marker as you add them to the container so you will remember them.
Secure the cover tightly when using a plastic container. Freezer paper should be loosely rolled so the paint is not crushed. Fold the ends over and tape or staple them closed. Mark the storage unit with the marker so you will know that this is paint.
Store the wrapped paint in the freezer section of your refrigerator until you are ready to use it again. Remove it from the freezer and let the paint sit out for a while. Mix each pile with the palette knife.
Store the paint again using this method when you are finished painting. It will not hurt the paint and the paint will never develop a skin as long as it is stored correctly.