If you start off with the biggest pot and fill it with potting compost and then take the next size pot and just put it on top of the one you just filled and lightly press down, this will both firm the compost in the lower pot and also mark where the pot will eventually sit. There should be a part all the way around the lower pot that the second pot didn't touch and this is now where you need to put some plants, I used Busy Lizzies but you could use begonia's or sat Petunia's, trailing Lobelia would be good too. When this is done then you do the same thing again with the second pot first of all filling it with compost as with the first. If you planted the first pot right you should be able to just sit in down on top of the first pot without damaging the plants. See the picture below.
This next picture was taken later on in the summer and shows you how it all worked out.
All the best,