Hi Rob,
There are others on this site that are more experienced and competent than I but will give you my thoughts:
Using etched sides is the easiest area to start modelling outside RTR.
Having the correct tools makes the task more straight forward, so an assortment of drills, files, cutting knifes + a plentiful supply of blades, razor saw, epoxy glue, fillers are the main ones you will need.
I think it is essential to have good pictures of the real thing, it make it so much easier to have something to check with. Great pictures on this site of the Mk3 EGV
I put the etched side against the stripped down doner coach and mark with felt pen the new windows. Then it is a matter of cutting them out, sticking the etched sides to the coach and follow the instructions on drilling holes for hand rails etc.
On the EGV the roof details is good on the SSM kit and it’s a matter of drilling and cutting holes in the roof and cutting the roof ridges to fit on the lifting panel etches.
I started on etched kits and each one you do is better than the last so the only way is to have a go. Any mistakes can be covered by fillers and when it is painted and decals added only you will notice where they are.
So have a go, the hardest thing for me was cutting into a good coach, after that it was straight forward.
PM me if you come across any difficulty.