Glycerin is an alcohol so it is a good solvent, better than water but not quite as strong as PGA, pure grain alcohol so for more resinous herbs you will want to use PGA or put a third or half PGA into your glycerin to make the tincture. The process is much the same as a water or alcohol tincture. take your glass jar and fill it half way with the herbs and fill the jar to about a half inch from the top with glycerin, seal it and set it in the warmest place in your house (not higher than around 80F) for about 3 - 4 weeks. Glycerin is very thick and warming it will thin it out and speed up the extraction but if it is too warm it will also dissolve chlorophyll from green leaves, which you may not want. After 3 - 4 weeks, strain the liquid through your favorite straining material, I usually use coffee filters but it takes a while because of the viscosity of the glycerin.