I have noticed several people finding difficulties in feeding cable harness' through the fabric guides. This can be so easy.
Find some plastic banding; the stuff that they wrap round large heavy packages. It comes in a variety of thickness' and widths, a look through shop packaging waste, skips, any industrial supplier will just throw it out. It has a thousand uses, and is brilliant stuff.. It has longitudinal fibres imbedded in the plastic, and can easy be split longitudinally to the desired width.
I used a single piece, stripped down to about 6mm, and about 500mm long. As someone rightly posted, if a cable has numerous connectors on it, then neatly, carefully, and with the smallest profile, 'nest' these together in line, holding them in place with masking tape, neatly working from the end connector toward the main cable.. Then use a small amount of tape to affix the prepared end to the strapping strip, already prepared.
The 'free' end of this 'assembly' is fed through the guides, straight through the base of the machine. As you pull this further, the other end of the strapping with the connectors attached, can be gently guided into the cable sleeve; gently hold existing cables from being dragged too, and ensure nothing 'binds' as the strapping is pulled through.
This method also helps to ensure that there is no damage, or stress, to the often sensitive cable terminations. If things get tight, either re insert, or just lightly squeeze the cable guide to 'settle' existing cables. Do not forget to support existing cables, so they are not being pulled as well, and stressed.
So effortless, and all that is left to do is carefully remove the tape..