You may not need to. Put the 60P clips in a 25fps comp and render a test or step through the comp one frame at a time and see how things look. Frame blending on and frame blending off. The conversion may be a waste of time. There is a caveat.
If you are doing a bunch of manipulation with the 60p footage like roto, keying, camera tracking, then do all of that before you incorporate the footage in your 25fps comp. The workflow depends on the project, but generally, all heavy manipulation of video should be done at the native frame rate and if the footage is interlaced the fields should be separated so you are working with as much original data as you can. You then assemble the comp for final rendering. Run some test to determine whether or not the shot benefits from using Twixtor.
