Step 1 of 4Gather the childhood evidence first
ADHD is developmental, so assessors look for the pattern before roughly age twelve. School reports are gold, not the grades, the comments: bright but does not apply herself, talks in class, careless mistakes, in a world of his own. A parent's memories serve the same purpose, and one honest phone call to a parent often beats a folder of paperwork.
If the paper trail is gone, reconstruct honestly: the phrases teachers used, the report-card sentence your family still jokes about, what homework nights actually looked like. Assessors know most adults do not arrive with documents, and your account of childhood is evidence too.
While you are in the past, note anything that mimics or complicates the picture: sleep problems, thyroid issues, substances, head injuries, and mental health history. Volunteering these strengthens the assessment; a clinician who rules things out is doing the job properly, and the what a proper report contains entry shows why that matters.