I use ceramic molds only to slump/sag my glass into dishes as I don't trust stainless steel molds not to crack the glass. This is because the mold heats up and cools off at a different rate to the glass (coefficiency) and stainless steel can be notorious for both cracking glass and for being difficult to kiln wash too.
So here's a selection of molds (moulds) that I use. All have been bought commercially and then coated with several layers of kiln wash before use. I use a haik brush to apply each layer of kilnwash, drying with a hairdryer in between layers. After a few firings the kiln wash needs reapplying. I use a haik for two reasons - one is that I like the textured surface the glass acquires and secondly, I get mad at the spray bottle when it clogs with kiln wash!
You can see by the color of the mold shown above that it has been primed with kiln wash but not yet fired in the kiln. After the first firing it loses its pink color and becomes white. The holes are to let the heat at the base of the glass escape, avoiding cracking. I love the shape of this mold, it makes the glass look as if the corners have been turned up.
Of course, haik brushes have their own frustrations - they shed long thin hairs in the kiln wash which should be brushed off quickly before applying the next layer. When the mold is bone dry the fused glass tile is placed on top of the mold and put through a fusing cycle. Once the glass reaches the temperature of 1400F the glass should slump nicely into the mold. General advice is that the slumped glass doesn't have to go through a "resting" temperature but I am a very cautious glass artist so I rest the temperature for 30 minutes at 1000F.
Going back to glass temperatures, because ceramic and stainless steel molds shrink at different temperatures it is easier to slump glass into a mold than to sag or drape glass or a mold. This technique can sometimes result in cracking. But don't let that put you off - lots of glass artists drape glass over stainless steel molds, otherwise we wouldn't see so many beautiful handkerchief dishes and candle holders for sale.
If you do choose to use a stainless steel mold then the best way to apply the kiln wash is to heat the mold in an oven and then spray with kiln wash whilst the metal surface is still hot. Otherwise the kiln wash just runs down the steep sides and makes a puddle at the base.
Squiggly Blue Glass Dish. All glass shown here is for sale at Glassprimitif on Etsy.