I wrote a few useful AviSynth functions for:
import("abarryAviSynthFunctions.avs")
goproFormat = AviSource("GOPR0099.avi")
framerate = goproFormat.framerate
audiorate = goproFormat.AudioRate
width = goproFormat.width
height = goproFormat.height
othervid = AviSource("othervid.avi")
othervid = ConvertFormat(othervid, width, height, framerate, audiorate)
vid1 = AviSource("GOPR0098.avi")
# trim to the point that you want
vid1 = Trim(vid1, 2484, 2742)
# make frames 123-190 slow-mo, maximum slow-mo factor is 0.25, use 15 frames to transition from 1x to 0.25x.
vid1 = TransitionSlowMo(vid1, 123, 190, 0.25, 15)
03/5/14