The question we get most often from managing partners is “where do we begin?”. The answer is rarely what they hope for. It is not the advice, not the client meetings and not what is unique to the firm. It is what is the same for every client.
Three criteria
Pick the process that is the most frequent, the most boring and the least judgement-based. Bank reconciliation is a typical answer. Chasing missing vouchers is another. Both happen every month for every client, both have known rules, and nobody in the firm gets better at them by doing them for the hundredth time.
And what you should wait with
Everything that requires knowing the client: assessments, advice, the difficult phone call. That is where the firm's value lies, and that is where the hours should move, not disappear.
Measure the time before you begin. Without that number nobody can say whether it was worth it.