Many car owners do some maintenance work themselves but never document it. Later it becomes difficult to remember exactly what was done, which parts were used and when the same job should be considered again.
A good DIY entry starts with the same basics as workshop maintenance: date, mileage, work performed and parts used. If you also add a part number or oil specification, the record becomes much more valuable.
Photos and purchase receipts add useful proof. You do not need dozens of them, but a couple of clear before-and-after pictures can be very helpful later both for yourself and for a future buyer.
The most important part is consistency. If every self-performed service gets recorded, the car history will not create the false impression that nothing happened between workshop visits.