i've only made little stepping stones but here's how i do it.

i use the indirect method for this. i glue my glass pieces upside down onto a sheet of craft paper (which i've cut to fit nicely in the bottom of the mould) and, once it's dry, i set it in the bottom of my mould after wiping the mould with vaseline so nothing sticks. the paper, itself, is touching the mould and the glass is on top. then i pour the cement over top of this and let that dry for a few days. to keep it from drying too fast, i put saran wrap over the entire thing and then spray it with water every so often. the slower cement dries, the less prone it is to cracking.

because your stepping stone is going to be so big (and i'm assuming thick, as well) you might want to reinforce your cement with something like chicken wire. pour half of your cement into the mould, then a layer of chicken wire cut slightly smaller than the mould, then the rest of your cement.

once it's had a few days to set, i flip it upside down and the whole thing pops out. i use a wet sponge to wet the paper and it comes off, exposing the design. what's nice about doing it this way is that the glass is flush with the cement, so you could step on it in bare feet and not get cuts.

if i'm reading you right, you're wanting to put your tesserae on top of wet cement. i guess you could do it this way if you poured basically almost all of your cement a few days ahead of time, let it dry, and then added just a thin layer on top when you go to arrange your pieces - but it won't be flush if that's what you're looking for. i also don't know about working directly in cement - i've always used grout or thinset, etc. just make sure to where gloves if you do it. cement can really burn.

i wouldn't recommend pouring all of your cement into the mould and then arranging your tesserae on the top at that time. it would just sink and shift, etc.

i'm sure you'll get a variety of responses but this is what works for me. good luck!

here's a link of how to do the indirect method just in case i wasn't really clear with my directions (it makes sense in my head - honest!!! lol)

Here is a link that might be useful: indirect method link

Hmmm, that's gonna be one heavy stepping stone... I would see if I could find one already made like at a statuary place, and just top it with your goodies..

I would put rebar in it also .It might break with out it I would rather be safe than sorry. Just to much work to see it crack.