Well i will start off with, if you only need twenty of one type of plant and there are 180 seeds in the packet don't sow them all at once.
Just sow 30, pick the strongst and allow a couple of spares for any later disasters.
If you do this you will not end up with trays of plants that haven't grown well, and end up getting planted out too close together with the end result being very disappointing.
I'm sure we can put a good list of tips together between us.
