A margarine tub with two or three layers of toilet roll and then a sheet of paper. Wet thoroughly then put in the carrot seed, put the lid on and put it somewhere warm. Check after two or three days for the little white bump that shows germination is starting. The sheet of paper is in case I am a bit late, the baby root will grow into the tissue if it is not there.
Onion sets I simply plant out. Bury them just below the surface. They will push up as they start to grow, but if they are showing before the roots have started to develop birds will tweak them out of the ground.