Here is a link to my Youtube tutorial but if you want a quick rundown read below:
MIXING YOUR OWN MEDIUMS
The Rule of Mediums is LEAN TO FAT.
At the beginning of the painting, use a lean medium to block in large areas and to help spread and blend the paint.
As you progress through the work, the medium should become fatter.
And finally, transparent glaze mediums are used at the end of the work.
1 part gum turps to 1 part linseed oil.
1 part gum turps to 2 parts linseed oil.
Equal parts of gum turps, linseed oil and damar varnish.