In reply to jamesg85:
I common mistake is to fit a new chain on a worn cassette. If that’s the case, they’ll not so with each other and will skip under load, more than the old cassette did. If this is the case, then it’s likely that the cassette also needs replacing. If you can, try a mate’s wheel (same type, manufucaturer, number of gears etc).
Another typical issue is fitting a new chain at full length, without considering how long it needed to be. This would lead to less tension on the rear mech, which can also lead to skipping (and gears jumping around), I imagine there will be a ton of videos on YouTube on how to get the chain length correct.