just some quick points, it all depends on how formal you want to be. Some are happy with a spit and handshake!
Find out as much as you can about your client. After you've had a look do a formal quotation and get signed acceptance you will need to ensure who you are working for, and what your terms of business are. This is really important because you need to know who is responsible. If you consider there is a risk suggest you invoice the company direct and pay your contact a comission.
Make clear your expectations and concerns.
make your terms and conditions clear and add "all work is subject to our terms and conditions, these terms may not be altered either by order or verbally unless agreed in writing and signed by........... prior to work commencing"
Ask if you will need to do a risk assessment, might be worth doing it anyway.
Be prepared to wait for payment. When ever i've sub contracted work out i consider it's my responsibility to pay the sub, and I do, even before i've been paid if that's whats agreed. It's your descison and is a judgment call.
It may be considered over cautious but when you are dealing with large sums it's always best to get it straight from the start so everyone knows where they are.
Don't proceed if you're not happy. Nothing worse than doing a job you wish you had not taken on.
As long as it's all amicable you will be fine.
Good Luck
