Hi

It shoudl tell you on the pack what thickness (I think they call it a 'tog') goes with what tempretures. The cheaper ones don't, just pick the same fabric ou would use as a blanket (probably a cotton or flannel), remembering they'll have clothes on under it.

Just wanted to be really pedantic and throw in that doonas and thick, heavy blankets can be a SIDS risk and aren't recommended until bubs is at least one (and I know plenty of people have used them and had no probs, just wanted to put the info out there).