There are no silly questions!
The answer to this is simple: Adjust the open, high and low according to the same factor as the close to the adjusted close.
eg. OHLC are 10,11,9,10. Adjusted Close is 5. Factor is therefore 10/5 = 0.5. Multiply each OHL by 0.5 for the adjusted OHL.
This assumes that the "Adjusted Close" is adjusted for all events (including splits, reverse splits, stock dividends, spinoffs, corporate restructures, rights issues, trading currency changes, special distributions/dividends,capital gains payments, liquidation payments, and normal distributions/dividends).
If your data is not adjusted for all such events then you do have a big job on your hands obtaining the exact details of each event, calculating a "dilution factor" for each one and applying it to the original (unadjusted) data. That's precisely what a data vendor does - you need to check the documentation carefully.