I am far from being an Expert, but from what I've picked up recently, you want to start with the best quality felt you can get, some seems to only last a few years whereas some seems likely to last a couple or three decades. You also want to get some "roof felt glue" (it probably has a proper name :?: To seal the joints with. And then a lot of short (13mm) galvanised clout nails (and several spare finger tips (hammer) ).
There are two ways to lay the felt, horizontally and up and over (vertically?). I think horizontally is probably best as it leaves fewer joints and the joints seem more water-tight to my mind. I think the up and over method is easier if you're doing it single handed.
Check for any rot etc in the old roof and make good, then lay from the lower edge up, leaving an inch or two overlap at the bottom and sides. One edge of the felt should have a strip without grit which is where you overlap the next layer and apply the sealant, this should be along the top. Nail about 18" apart along the top edge only, where the rolls will overlap. When you get to the ridge, be prepared to curse, jump up and down and cry. It can be a right *****r to cross the ridge without it buckling and going off at angles etc. Ideally you want to overlap the ridge by about 4 inches with the whole width of the felt. You also want the side you're most going to Look at, look the best, so the last roll that goes on should go along this side, leaving the dodgy looking join where someone else can look at it. You may need to cut along the length of the felt roll to give you the best overlap at this point. Be prepared to use several hundred Stanley Knife blades or Industrial Snips.
Finally tack the overlap around the edges with nails about 2" apart. Though this can be fiddly depending on any overhang, facias etc.
Also see here;
http://www.ikogroup.co.uk/SiteFiles/Con ... to-_2_.pdf
