If you want to back or lay at less than the minimum $6 it's quite easy. Instructions are at the following link;

http://www.probabilitytheory.info/topics/minimum_stake.htm

I've found the whole site very useful and one of the not so many that I'd recommend.

The author Peter Webb has the very rare skill of imparting important mathematical information in a way that can be readily understood by most.

It would do most of you a lot of good to spend some time there.