In my experience, this is something you can learn as you go.

To get my first major gig started, a 1-page proposal that outlined how my services would help the client fulfill their goals was enough. A formal contract came later and we have been working together for a year now.

In fact, the client was asking "Where do I sign?" before I even mentioned the formal proposal.

What I learned was that the right clients care about their results and whether you can get them there first and foremost. Pricing and contracts are afterthoughts.

In general, most clients use a template contract. An accountant and a lawyer are going to be required if you become serious about consulting.