It may be worth buying a really cheap boxset from ebay, a market stall, charity shop or similar - just to get hold of a boxset to carefully take apart, see how it was put together, the measurements, the folds and tabs - just to practice with / on.
May be an idea to post a couple of pics of your boxset to see how you’re getting on? - and for others who’ve already completed theirs to give some pointers / advice etc
Good luck with it 😃