In hair care, glycerin works by helping the hair shaft retain moisture on the surface. When mixed into a restorative hair mask or conditioner, it may make dry-looking lengths appear smoother, softer and less prone to frizz. It is most relevant for the mid-lengths and ends, rather than the scalp, especially if your concern is dryness in the hair fibre.
A practical way to use it is to mix a few drops into the amount of hair mask or conditioner you are about to apply. Blend it thoroughly, distribute through damp lengths, leave the product on according to the instructions of the mask or conditioner, then rinse if that product is rinse-off. Avoid adding a large amount directly to the whole jar unless you are confident in your formula-making, because changing a finished product can affect its texture and how pleasant it is to use.
If your hair feels sticky, heavy or coated after use, reduce the amount next time or use it less often. Glycerin is not a heavy oil and does not leave a greasy film, but too much humectant in a hair product can still feel tacky. If you have a very sensitive scalp, do a small patch test or keep the mixture mainly on the hair lengths.