I found an instructional thing in a candle book recently for how to do this. You use hurricane wax and pour a pillar/square/other large candle like your normally would. Don't worry about the second pour, you won't need it. After the top and sides have a thick skin (not hard, just skin) you cut a hole in the top leaving just a "crust" around the edges. Pour out the still liquid wax, save it, then press your materials (stamps, pressed flowers, etc...) into the inside, let cool completely. Now, take some of that leftover wax and pour it back in and repeat the cutting/pouring steps so that you now have embedded your material. I haven't tried it yet, but it makes sense and seems simple enough.