Most home and vehicle jobs can be split into steps. Some steps can be done alone. Others need a second person to hold a part, steady a ladder, read a gauge while the other hand works, or lift a component that is too heavy for one person. Split the job before you decide who does what.
Write the job as steps
Write the job out as steps, one line each, in the order they happen. Mark every line that needs a second person, a lift, a tool you do not own, or a trade.
Questions for each marked step
Ask whether the step needs a trade, a lift, or a tool you would have to rent or buy. Ask whether the job can be stopped midway without leaving the item unsafe or unusable. Ask whether you can return the item to its original state if you change your mind.
Ask the contract
Read the paper for any line about who may perform the work, and whether the contract requires a specific shop, a licensed trade, or a written record of the work. Do not assume a step is allowed because you can physically do it. Write the answer beside each marked step.
Decide per step, not per job
A job can be divided. You may do the steps that need no help and hire out the steps that need a second person or a trade. Record which steps were hired out, who did them, and the date each was done, and keep that record in the same folder as the contract.
What to keep
Keep the step list, the answers from the contract, and any receipts for parts and labor. If a claim is ever raised, these papers show which step was done, and by whom.