Replacing rotted soffit
 

http://i59.photobucket.com/albums/g3...own/soffit.jpg

Not my house, just a picture.

From the face of the exterior to the tip of the soffit. How to replace that piece of truss?

What methods do I use?

Measuring, sawing, screwing/nailing, and painting ought to do it for you.

No other ways?

Do they sell ties for this application?

If you are talking about tails of the rafters, they do have to be strong enough to support that area of the roof. If you are removing 24 inches you will need to replace it with 72 inches along side the rafter and attach to the rafter inside the attic, lots of nails or screws.

I totally agree with Neal on this. Remove the fascia and slide new rafter in. Would advise you use some construction adhesive as well.