The ability to quickly and efficiently set up an equalised anchor at the top of a pitch is a skill that can save hours on long multi-pitch routes and the Sliding X system, if used correctly is one of the fastest set-ups I know. As is usual with all things climbing, much debate over its safety and appropriateness can be found online and it is worth having a read through the different opinions so that you are aware of them.
It can be constructed from a normal Nylon or Dyneema sling, but personally I don’t recommend this approach. The drawback of the Sliding X is that it can shock load the anchors and whatever you’re using to tie it with in the event of an anchor failure (see this video for why you should never shock load a sling). I use either 7mm or 8mm cord, tied in a large loop, usually around 2m long, so a total length of about 4m-4.5m.
A limiter knot should be tied at either end (an overhand works great) to minimise the chance of shock loading and also as an extra back up should one end of your Sliding X snap or be cut. The further away from the anchor that you place your limiter knot, the lower the chance of shock load in the event of an anchor failure. The trick is to work out how much movement you’ll require at the belay and then this will dictate where you place your limiter knot.
When you clip into the X make sure that you make a twist in one section and then clip into the second section (this is what creates the Sliding X). Run the attachment karabiner up and down the X to make sure it’s functioning properly and that all is tied correctly and of Of course stick to the 60 degree rule!
This system works brilliantly for multi-pitch sport climbs when each belay station is made up of two bolts. The system is of course only as good as the anchors it is attached to, and you’ll need to make the call on whether your individual situation requires more than two anchors, in which case a different set up should be used.