General Tips & Troubleshooting:
1) REMOVE the thin black (conductive) skin from around the white insulation. If it touches where the conductor needs to enter, you’ll create a complete short or partial short with weak signal
2) Attach the first plug to your longer supply of cable before cutting to length. Measure length required to the other plug. Cut, attach the second plug to complete cable. Attach next plug to long supply of cable. Rinse. Repeat.
3) You need the cable to slide in completely with a snug fit. If you have trouble pushing the cable into the plug, don’t twist the shield as much.
4) Wear rubber gloves to get a good grip when closing tight. I can close 10 quickly without complaining. So can most guitar players. More than that you might want those gloves.
Straight SIS Plugs:
Assembly of the Straight SIS plugs is somewhat similar. You hold the cable against the plug and remove the jacket at the black ring.
Remove the black skin covering the white insulation, and cut away about half the length of the shield after you twist the strands together.
Wrap the shield around the white insulation away from you, so that it is no thicker than the cable jacket, and remove about 3 mm of white insulation to expose the core.
Push the cable into the plug and when you seat the core, rotate the plug about 10-12 full revolutions. This will simultaneously thread the copper core into the tip of the plug and screw the body of the plug onto the cable jacket. Depending how long you’ve left the white part, you may need a couple more or less turns to seat the cable jacket inside the plug. You only need the jacket to enter the plug a few millimeters.
This video offers a guide: