OK, so there are two ways to go here in terms of making glair into a gilding size...
The first is called (fittingly) putrid glair. Just let the fresh glair sit around for a while (4-7 days, depending on season and general room temperature). It will change color, darkening, and unfortunately begin to smell. At this point pour it off into a coverable container and store it in the fridge. This keeps for a long time, and seems to get more effective the longer it sits.
Fresh Glair can be used as a gilding size by adding sugar to it. Measure out 4 parts glair.
Once you've got the 4 parts glair measured out, add one part finely powdered sugar. Now, many people don't know this (I certainly didn't...) but Confectioners sugar or powdered sugar is almost always some part corn starch. In order to get pure powdered sugar, one has to grind ones own.
Once you have ground the sugar, measure out one part.
And add it to the glair.
Mix until all the sugar has dissolved.
Decant this into a coverable container, store in the fridge. Allow to warm to room temperature before applying.