You can do either method; probably simpler to put all the layers on at once (making sure the largest pieces of glass are on the bottom).  Just take extra time to make sure the resin fully saturates all of them.  I was a bit skeptical at first that they'd all get wetted out when I did it, but it worked fine. 

 If you choose to do one layer at a time, you can put subsequent layers on as soon as the epoxy has set so it's no longer tacky (that way you don't have to sand between coats).   If you put it on too soon, the glass will stick to the tacky layer and you won't be able to get it flat.  If you wait more than a day or so you'll have to sand to get mechanical adhesion between the layers (chemical adhesion works if it's just a day between coats).

Thanks for your reply...I guess my kit is not designed to provide extra layers, but if it is cut correctly I understand that I will likely have enough for extra coverage..and I will do it all together...TKS