I use potting soil, chicken grit and a bit of perlite. (actually the potting soil I get has perlite in it, and I ran out of mine, so if you know your soil has perlite I wouldn't worry about adding more) then I add about a quarter part chicken grit. Mix that up and bust up any larger chunks and put the baby semp on the surface - if it has roots I twirl it around until the roots have buried themselves, then I top dress using the chicken grit - which is better at keeping the plant elevated off the soil!!
I'm not at home so I haven't seen my semp beds for a few days...get to go home for a while today, and can't wait!!! Want to see how they're doing after all this hard rain! I'm finding that I no longer need to worry about them now that I have them top dressed!
Its seriously the best thing I've found for my plants!!