OK, I'm a bit of an old hand at this now.
You need to book the ASSISTANCE in advance, I was actually advised to still buy tickets on the day in case the machines to pick up pre booked tickets were not working. To book assistance you need to know what station you're travelling from, what trains you'd like to travel on (times etc) and where you're going to. Then you ring the assisted travel line for the train operator who runs your starting station, so for me, even if me and my friend H are going on CrossCountry trains, I have to ring Virgin as they're the main provider for our local station. They'll go through what help they can offer, take your travel details, and arrange it all.
On the day, you need to be at the station half an hour before your train, buy your tickets, then make yourselves known to a member of staff. When the train comes, they bring ramps etc, help you on (they've always pushed my friend's wheelchair up the ramp and to the wheelchair space for me, without being asked) and get you settled. They'll phone your arrival station and when you get there, there will be someone to help you off the train.
To go home, again arrive half an hour early and make yourself known to staff, and they'll do the same as above but for your journey home.
They need at least 24 hours notice so they can get staff etc, make sure there's spaces on the train if you're taking a wheelchair and sort out anything else.