A spreadsheet is the right answer until two people need it at once. Here is what changes when the same information lives in something built for a garage.
Nothing wrong with a spreadsheet. It is just not built to be a garage's memory.
This is the easiest move there is, because the importer wants exactly what you already have.
You already have the data. Copy your customer and vehicle sheet, and put the columns in the order the Pistonlog template uses. If you keep one row per car with the owner repeated, that is exactly the shape it expects.
Download the template and paste your list into it. Customer name, business name, phone, email and address, then the registration, make, model, year, fuel, MOT date and mileage for each vehicle. Anything you have not got, leave blank.
Upload it and the server works out what it would do, then shows you. New customer, matched to one already in Pistonlog, or unclear. Nothing is written yet. Anything unclear stops the import rather than guessing which record you meant.
When the preview looks right, commit. It all goes in as one lot, so you never end up half imported. If it turns out wrong you have 24 hours to roll the whole thing back.
Do not delete anything. Leave the file where it is, work out of Pistonlog, and check back if something looks missing. After two weeks nobody opens it, and you can archive it knowing you did not need it.
That is the only bit most garages dread, and it is the bit you have already finished.
The things garage owners actually ask, answered straight.
Set up an account, run a couple of real jobs through it, and only bring your customers across once you know it suits your garage.