Look on the DVLA website and you should find it there. I have registered several vehicles over the years so unless it has changed drastically it should be fairly straightforward and is all done at your local DVLA office (not Post Office).
You will need a proof of purchase, proof of year of manufacture and/or first registration plus a current MOT & (I think) insurance. The MOT & insurance are initially obtained against the frame number and then when it has been formally registered you get the doccumentation changed. You get the tax disc at the same time as being given the new registration number.
Most of mine were ex-military vehicles and they come with release papers that proved when it was first put into service which legitimately allows you to have the year set as either when it was made or when it was first put into service (which could actually be several years after it was manufactured as many brand new military vehicles are put into storage for varying lengths of time). As the first registered keeper (after the MOD) the new logbooks showed the number of previous keepers as nil (not bad on 20-year old vehicles!). I also I had one of my Laverda's re-registered as it had sat in the showroom unregistered for 2-years and the letter denoting the year on the number plate looked wrong for that model so 20-years later I had it re-registered against the year it was actually manufactured (the importer provided these details for me).