Fid SetYou need a special fid set for doublebraid, not the one called a Swedish Fid set. The fids have to be slightly smaller than the rope you are splicing, so you need a set that copes with the different thicknesses of rope you are going to splice.
I use 12mm for long lines at 30ft, 14mm for a 24ft in-hand line, and 16mm for a 12ft short in-hand line.
Any heavier and my shoulders ache using them. I have never tried a halter though, you would probably need something thinner for this.
Ps. HH I have been looking for the instructions, think they are on the PC at work, I will dig them out.