It's easy - ish!
I'd advise buying new hubs - probably only a couple of £ more expensive than bearings on their own. It's what I did last time.
It's not too difficult a job, just messy!
1) Take off hub cap
2) Remove slit pin from shaft
3) Unscrew nut
4) Pull off hub (back bearing may come off with it, or may be left behind, in which case take it off).
5) Pack new hub with grease
6) Grease u the bearings (best done using fingers, and work into rollers on bearings)
7) Put on rear bearing (make sure it's the right way round!)
8) Put hub on
9) put on front bearing
10) Screw nut back on, tight, then backed off enough to line up the split pin through the hole in the shaft (should be about 1/4 of a turn if it was tight enough in the first place)
11) Refit split pin
12) Fit wheel back on, spin, check tight enough without binding.
13) If 12 seems OK, remove jack, tighten up wheel nuts, jack up again
14) "Wobble" wheel from side to side to make sure play is not excessive (should be a tiny bit or will bind)
15) Remove jack and job done.
No special tools or pullers normally required, just some common sense.
This is as good as I can remember. No doubt it varies a bit on different makes of trailer, and others may well add to or disagree with some of the above, but it's how we've always done it.
Make sure you have the right type of grease - GT will tell you what's suitable.
Hope this helps