You don't NEED a contract or anything like that, they are just very good practice when inevitably with rifles you need to deal with the police.
You only need permission to shoot on the land from the landowner, if they say 'yeah sure go ahead' then your all good to go.. but then proving that to anyone else can be tricky.
Here is a thread where a decent generic form was laid out, there is no standard form but something along these lines will work
http://www.thehuntinglife.com/forums/topic/40744-my-shooting-permission-forms-for-you-to-use/
Just a simple form saying who you are,