Never done this before and would like to know where to start and what tools would be needed.The rest of the frames are sound (TwoExtreior sills both 6ft long.)

I have very little experience of extrerior wood repairs Could you please help and advise any essential tools.
Thank you!:)

Kind of hard to do with out taking window out. If interior portion is solid, I suggest having siding man cover with aluminum.

I have actually done this in an old (1922) house I owned. It was my practice house. The house and I both needed a lot of work! Anyway, you don't say if this is rotted on the inside sill or outside trim? If it is like the home I had the outside would just be a trim board that could be pryed off and replaced. If it is an inside sill you should be able to raise the window and knock the sill out toward the inside of the house with a hammer. When I replaced mine I took the old wood to a lumber company that specialized in hardwoods to match as close as possible to the original wood which turned out to be yellow pine. Hope this helps. Please mail back with what exactly you are replacing.

Some pics would help.