Step 1: Fix the person
Write down the full name, any service number, the service if you know it, and the dates you can prove. Keep what you suspect in a separate list from what you can prove. A name alone matches too many people, so every added filter, a number, a unit, a firm date, makes the next search sharper.
Step 2: Find the service record
Use the service guides for RAF, Army or Navy to decide which holder is likely for the dates in question, then search that holder's catalogue. The service record is the spine: postings, ranks or ratings, trades, and discharge or casualty details, all dated.
Step 3: Follow the postings
Build a dated list of postings from the service record, in date order, with a source note for each line. Do not write a narrative yet. A dated list exposes gaps, transfers and renumbering that a narrative would smooth over.
Step 4: Add the unit or ship record
For each posting period, open the matching unit record: an RAF operations record book, an Army war diary, or a ship's log. Remember that these are unit records, not personal records. They show what the unit did while the person was posted to it, and presence needs both halves, posting plus unit action.
Step 5: Add medals and commemoration last
Check medal rolls and, where the person died in service, the commemoration entry. Medals prove entitlement under their own rules, and commemoration proves death and burial or memorial place. Neither replaces the service record, and both are stronger when matched to it.