Using a purified cold pressed, linseed oil, pour the oil into a shallow plate (about 5mm deep),  then this needs to be covered with a sheet of glass to keep dust out . The process works by oxidation  so the glass needs to be raised somehow to allow air in. Put on  a sunny windowsill, the more sun it gets the better. Air on the surface of the oil  and ultraviolet rays from the sun start a slow drying and thickening process. It is important to remember to stir the oil every day to prevent  a skin forming. After two or three weeks  the oil will be thicker and more viscous. Decant into  an appropriate air tight storage bottle This is  a great medium for plein air painting as it is half oxidized so dries quickly allowing  the painting to be worked on in thicker layers without it getting too oily and slippery. The oil yellows slightly in the process and dries with some shine. As with all new medium, it  needs  a little  practice to understand its  properties, and the viscous flow should  be adjusted by adding turpentine,  (or Zest-it!) to suit your painting process. ( a recipe from  "The Materials of the Artist" by Max Doerner.  - quote: Cennini calls it the best of all oils "I could not give you anything better")