Many people use glue to hold their arrangements of glass together to get it to the kiln. There are many kinds of glue that can be used. It is best to avoid resin based adhesives, but most other kinds of glue can be used – including hair spray, lacquer, super glue, CMC and PVA in addition to the proprietary fusing glues. The cheapest with the fewest additives seem to get good results.
Remember the glue burns away long before the glass becomes sticky, so if the glass won't stay in place while you are assembling it, it won't in the kiln either. The glue is only to keep things together while being transported to the kiln.
But this note is about were to apply the glue you choose to use.
The glue should always be used in minimum amounts. If it is a strong water based glue, such as PVA, it can be diluted with water and still provide sufficient adhesion. The glue should be runny, not thick or a gel. Unless the adhesive is a spray, a small dot at the edge of the piece to be glued will be sufficient. Capillary action will draw enough glue under the piece to stick it to the base glass.
If you are spraying the adhesive, that should be done at the end of assembly, to avoid flooding the base glass with adhesive. It is often best when using these lacquer based adhesives to spray a small amount of liquid into a container and use tooth picks or other pointed implement to dot the lacquer at the edge of the pieces to be attached. This way you can glue as you assemble rather than waiting to the end.
Adhesive under the middle of a piece of glass is likely to give black marks and even large bubbles, as the combustion gasses cannot get out from under the glass. So always confine your glueing to the edges of the pieces. A dot at each end is all that is required.