I need to do the same thing with mine and there are some useful videos online. Looks like you just need to cut the wood past any rot, into good wood, and replace with new wood and filler, sand, prime and paint.
Some of the seals around my timber window frames are falling apart but I'm unsure what materials to use... Looks like the original builders have mixed sand or grit into something and packed it in. Could I just use caulk? Any advice much appreciated!