Fix the scope first. For most blocks of flats that means a Type 1 FRA: non-destructive, common parts only, with flat entrance doors sampled from the common side; the FRA types overview explains when a wider scope earns its extra cost. Then shortlist assessors whose competence you can verify rather than take on trust: BAFE SP205 registration, FRACS certification or an IFE register listing are the third-party checks that carry weight, and because choosing a competent assessor is now part of the legal duty, keep a note of the checks you ran. Ask each candidate for a sample report and judge the reasoning, not the cover.
Send the assessor the building file before the visit: floor plans, the previous assessment if one exists, fire alarm and emergency lighting service records, door inspection records, external wall information for taller buildings, and anything known about residents who would need help to escape. Good records shorten the visit and sharpen the findings; missing ones surface as assumptions and caveats in the report, and caveats are where enforcement questions start.