Mix one part linseed stand oil with two parts turpentine or odorless mineral spirits. You will find that these two liquids won't mix readily. Linseed oil can be very thick and slow drying depending on the temperature and humidity that you are painting in. You can substitute sun-thickened linseed oil for the linseed stand oil in order to save a few extra days during your painting's drying period. However, sun-thickened linseed oil may cause the lighter oil colors you are using to turn slightly yellow over time.
Cover tightly. Use a glass container with a screw cap to get a tight seal.
Let it sit. You may need to wait several days for the linseed stand oil and the turpentine or mineral spirits to mix completely. Turning your glass container on its side or top several times a day will help the liquids mix together more quickly.
Pour a small amount into another container after the linseed stand oil and turpentine or mineral spirits have thoroughly mixed together. You are now ready to dip your brush in your oil painting medium and begin your new masterpiece. Be sure your container has a tight fitting lid in order to save any mixture that may be left over after your painting session. You can purchase oil painting medium at any art supply store. However, making your own is a wonderful part of the art process and will probably save you more than a few dollars.