Set your ground poles up in a fan arrangement. I use 8 ft poles and have the ends of the poles touching at one end and spaced out at the other end (like you would fan cards to see them in your hand). Then, as you ground drive them or long line them you can vary their stride depending on where on the circle they are...smaller circle, small stride...large circle, lengthened stride. Just remember to give them time to learn what ground poles are and how to use them. I try to start them out where they naturally stride (which will depend on each individual horse, tall, short, long bodied, short bodied will all affect their strides) then once they get the hang of it I open the circle up and make them lengthen some, just remember they have to build muscle to lengthen properly so a few circles lengthened at a time to start. Then smaller circles require collection and is even harder for them without the proper muscling.
I hope this makes sense, seems harder to explain than I thought it would.
Donna