I've been using the Grey/Yellow Milliput with Happy Seppuku stamps:
1. Place blob of milliput on base
2. Spread out milliput across entire base with wetted thumb
3. Flatten milliput against cutting mat
4. Press into mold
5. Remove and trim excess with sculpting tool
This is a pretty quick process, the water from my thumb prevents the Milliput from sticking to my cutting mat or the mold. Sometimes the release from the mold is not great, but I can always re-do the process quickly. I have been very happy with the results so far.