Yes, you will need an SOW, (statement of work) that articulates very clearly what you aim to accomplish. Near the latter part you should have another section ( there is more than one way to accomplish this however ) that clearly states what the "acceptance criteria" is for meeting the goals - ie - spells out usually in bullet points reachable goals for the project that when met everyone agrees your end of the deal is met.

Along with this it should also cover your rates, payment schedule, terms, and what things cost if they fall out of scope.

Finally I find it useful to add a "prerequisites", and "disclaimers" section that say that work can't begin until you have things like write access to their git repo or access to their aws account ( or whatever ) and that the current SOW doesn't cover getting it into the App Store or bugs that come after the acceptance criteria are met.

Put places on this document for your company or self, and the other party and make sure you properly distribute and record the signed docs.

IANAL but have one, and the more you get into consulting and bigger contracts it's good to run things by a lawyer if it's something that's high value, with a lot of effort, or even remotely puts you at risk.

Be even more careful signing documents clients give you;)

Have fun