I'm hardly an expert and I mostly coat handmade paper (not all that
successfully, as I've whined about on the list), but I did find coating glass to
be much easier than paper. I would heat the glass (or aluminum plate)
first, and then pour the emulsion onto it (with a catch tray underneath).
It requires a little fancy handwork, where you tip quickly to get all four
corners coated, but you'll get the knack of that pretty quickly. I
found that heating the emulsion more rather than less helped to me get an even
coating before the emulsion started to gel (warm glass helps with this
too). Then I would hold the plate level in my hands for a few more seconds
until the risk of "drippiness" had passed.