I have never thought of doing this before until I read about it and really it makes a lot of sense.   Here I have put wet seeds on to wet tissue paper and put it all in a clear plastic bag.   In just a day they started to sprout and by three days they are more than ready to be put either in potting compost of straight out in the garden.   As suggested the best thing to do is to cut a square of the tissue out and just put that with the sprouting seedling into the final position.    These are mixed salad leaf seeds which I planted outside at the same time.  The seedlings have all sprouted and are more than ready while outside there are just one or two starting to show.