I to learned to bevel by hand before buying a string cutter. Like others have said, secure one end well, wrap your finger in some leather to protect it and give some slide and hold a very sharp knife at a 45 degree angle to the edge of the leather as you walk backwards keeping the lace taunt. It is fairly easy to pick up once you do a string or two.
3 colors in a 4 strand braid is a little bit of a challenge, you would have to either tie off the ends with some sinew or tie a terminal knot but that would require four loose ends. When I do lanyards 1 or 2 colors I use a length of 4 strand and make the loops of the lanyard and bring the ends together and tie them together with sinew and cover with a fancy knot.
One other thought would be to cut your strands an inch or two long and fold them over and braid around the extra inch, you could even trim the lace down a little to 3/32.
Nice work on the lead!!
John