What to record
For every item: who has it, what it is (hard hat, harness, RPE, gloves, boots, hi-vis), size or model, the date issued, when it's next due an inspection and when it should be replaced. Add a note when something is returned, lost or damaged.
Set inspection intervals
Some PPE only needs a pre-use check; some needs regular detailed inspections by a competent person. Fall-arrest harnesses and lanyards are the usual example - follow the manufacturer's instructions and your risk assessment, and inspect more often where they're used hard.
Who it covers
Since 6 April 2022 the PPE at Work Regulations cover "limb (b)" workers - people who work for you on a casual or irregular basis - as well as employees. If the risk assessment says they need PPE, you provide it free of charge, keep it maintained and replaced, and make sure they know how to use it. The genuinely self-employed aren't covered by that change.
Review it regularly
Look at what's due or overdue each week, chase returns when people leave, and replace anything past its life. A register that isn't looked at is just a list.