Look at the BHS website, there is an excellent template which you can adapt as required. It is the one we used when I loaned Clyde and seems to cover everything.
Just remembered, what do you do with passports when you loan a horse out? Obviously if it goes with him then I can't prove he is mine if I have to, as they may change the ownership (doubt it but still a risk) yet they will need it if they transport him anywhere.
Keep it, but if they need to transport it anywhere lend it to them. Loaning has it's risks, but if they're nice people they probably won't even think of stealing him!