A rappel sling is designed to increase the amount of control you have over your abseil ropes and descender, limit mistakes in their attachment and removal, and make it quick and easy to safely connect oneself to belay anchors, and as such is a very important piece of gear on any multi abseil descent. The actual sling is just a standard climbing sling which modified once on the decedent and so requires no extra equipment. Take a 120cm/8 foot sling (a 60cm/4 foot will work but not nearly as well) and larks foot it directly into your harness belay loop or through the leg loops and waist if your harness has no belay loop. Larks footing slings is not recommenced normally but in this circumstance its ok. Next, tie a simple overhand knot about 30cm up the sling from your harness, and then another at the far end in order to form a small loop into which you can clip a screw gate.
Your descender is clipped into the initial 30cm loop you have formed. The advantages of this set up instead of using the existing belay loop are that it is much easier to visually check and monitor your descender, screwgate and rope, both during set up and in use. When wearing a lot of bulky clothing, or encumbered by a large rack, this system makes setting up far less fumberley and avoids nightmare scenarios where the descender is mistakenly clipped into the wrong part of the harness, or not into the harness at all due to the fact you can’t see your belay loop clearly! You are able to bring more force to bear on your breaking hand, as it is no longer blacked by your hip, and can be more easily positioned directly below the descender.
The small loop in the far end is used as your primary connection point to belay anchors. When descending the screwgate karabiner can be clipped into the descender’s karabiner in order to keep it out of the way yet keep it at hand, or clipped into the rope you’ll need to pull, both to remind you which one it is and to separate the strands and reduce twisting. Your backup prusik can either remain attached to your harness leg loop, or it can be switched to your belay loop (Leg loop?). The advantage of having it attached to the belay loop is that it’s in line with everything else, making it far more comfortable especially when wearing a heavy rucksack.
The sling can also be used to bounce test suspect belay points before committing to them. This is done by clipping either your top karabiner into the piece you want to test, or by clipping the top karabiner into the 30cm loop into which the descender is clipped. Once attached, lower down until all your weight is off the rope (you should be backed up by your prussic, plus you may want to back this up by tying in as well), and apply a gradually increasing static load onto the piece until you are satisfied the belay is sound.
Another variation on the rappel sling is to use a daisy chain, as this has far more adjustability built into it due to its myriad of sewn pockets. When doing so be aware never to clip two pockets at once with the same karabiners, as this cross clipping can easily result in the karabiner detaching itself if the bar tacks should fail.