How do I buy an e-ticket form my Norther Journey?
It's often worth using trainline for an intial search, as their website and app is far superior to any TOC offering, particularly if you're not used to looking for rail tickets. They will offer e-tickets on any journey that permits them (they aren't available on all journeys). They also offer split tickets and will do it without the need to apply any special requests or permissions.
In some circumstances they charge a modest booking fee, (but never for sameday travel), the fee also varies depending on if you're on the website or app, if you want to avoid this (if they're charging you one), simply note their ticket selection and buy it from any TOC website, none of whom charge any fees. LNER is probably the best, particularly if you want e-tickets. If there are no advance tickets in your journey, there's no advantage in buying before the day, only disadvantages, so it's best to wait till the day of travel. Of course, if Advance tickets are involved, this isn't always possible.
E-tickets from most (all?) providers can be downloaded to your device, stored in Google/Apple wallet, printed off, shown in the app or shown in the email. So they're very flexible. Just don't get confused and buy m-tickets! However, m-tickets are not offered by Trainline or LNER.