Hi, used to write these all the time. The rules on references are quite vague but the basic gist is you can't put in anything that you can't prove, or isn't an objective review. THis is for your protection rather than for the person.
I would normally start with confirming your relationship with the person, the length of time you have known them, what the job/placement was for, and objective comments on the performance. eg
Miss X is a placement student under my supervision at XYZ nursery. The placement was organised via ABC college as part of her Childcare NVQ (eg) and commenced in January of 2007.
The objective of the placement was to provide Miss X with practical experience of working with children aged 2 - 3 years etc, etc.
During her time with XYZ nursery, Miss X has completed all tasks required of her competently and attentively, and has proactively taken steps to apply the theoretical knowledge gained from her studies in a practical way. Always enthusiastic and enquiring, Miss X has integrated into the team well and we regularly receive positive comments on her performance from her peers and our clients.
Miss X's approach and manner with the children in her care has been exemplary and she has shown herself to be sensitive and compassionate at all times.
We hope that Miss X continues to develop her skills in childcare and would have no hesitation in welcoming her back to XYZ nursery should she continue to show potential.
Hope this makes sense - the bit about showing potential is a little bit of a caveat - she may be good now, and you would have her back if she was the same, but you are not commenting on her future skills IYSWIM.
Obviously everything I have personally written here is not based on my personal experience of the lady in question and I wouldn't dream of suggesting that you use this as is (!-my disclaimer!) but I hope it gives you a start! Put it on letter head, dated and with TO WHOM IT MAY CONCERN, Dear Sirs and RE: Miss X.