We start by reading what you have
Usually two weeks, always billed. We read the existing specification, the existing code and the last two years of decisions. Practices that skip this step are guessing, and the guess is always that your problem is the one they have solved before.
We write the constraint down before the design
One page. For this system it was "every vertical value is a multiple of 8px, no exceptions". Yours will be different and it will rule out things you wanted. If it does not rule anything out it is not a constraint.
We build the checker before the components
A rule that is not machine-checkable will be violated within a quarter and nobody will notice for a year. The audit script in this system was written before a single component was.
We hand over and leave
No retainer, no ongoing licence, no dependency on us. The system belongs to you including the checker, and about half our clients never speak to us again, which is the correct outcome.