I really wouldn't worry too much to be honest. Just have a read of "
Practice Guide 5". It's very "wordy" and a bit heavy going in places (as these things often are) but it really does tell you all you'll need to know, including what forms to complete and what accompanying statements are needed.
I guess the hardest part is the 12 years possession that will need to be demonstrated. In our case, I had fenced off the land some 15 years previously, and had photos from back then that demonstrated the time lapse by way of plant / tree growth and weathering on the new shed that was built. We also provided contact details of a neighbour who was willing to confirm the train of events. In the event (and I suspect as it was only a small parcel of land), Land Registry were happy to accept our "Statement of Truth" which has to accompany the application, and granted ownership and first registration without any issues.
Short version: we just took over the land, and some 15 years later claimed it as ours.
Cliff