My best advice is as follows:
1) Upfront payment before work commences. Say charge 50-70% and remainder on completion.
2) On final completion provide the deliverables with watermark if its graphics or if its a website on your own hosting e.g. host it temporarily as a subdomain. Once approved and final payment received provide the final product e.g. graphic without watermark or transfer/install website.
3) Prior to work commencing you need to send a simple SOW (statement of work) together with the cost and agreement on payment on milestones.
In addition you could also consider escrow services as a third party to hold the funds whilst work is under way... Personally I would only consider doing the work if the percentage upfront or all of it is paid before starting...
Regarding invoices... Well there are plenty examples online... just search for "invoice examples" and adapt it to fit your business i.e. add your company, vat details etc ... e.g.
Hope that helps!