It's been a long while since I tried tying one of these Celtic Crosses. I used some scrap lengths of olive drab 550 paracord and black 425 paracord, with a loop of 0.9mm cord as a hanger.
The last time I recall attempting one was after seeing knot tying friend, Ken, post an example on his blog back in 2009.
For that first time, instead of paracord, I'd used some cheap flimsy craft cord that felt like yarn and the knot kept twisting and collapsing when I was tightening up the slack, so I grew aggravated with it and put it on the 'to do later' list, which ended up being much later, lol.
The finished Celtic cross knot is flat, like a mat type knot, so it doesn't lend itself well to use as a lanyard or fob for a knife/flashlight/zipper pull, especially with larger diameter cord, but it does make a nice Christmas tree ornament, or hung from a cabinet pull, vehicle rear view mirror, or similar method of display. Done in smaller cord, it works well as a necklace pendant.
I'd tied my example from looking at a finished one, where it's not too difficult to see the pattern with two different contrasting cord colors, laying out one, then interweaving the other, then doubling and gradually working the slack out, taking care to keep the desired shape and not over tighten.
A quick search brought up a few online tutorials for the Celtic Cross, one here, another here, and then another. I'm not sure what book the Celtic cross may have originally been found in...
And I'd found an unfinished project, where I'd tied a long four bight Turk's head knot, doubled, with 3/32" tether cord over a straw, then tossed it in a storage tote a long time ago when I hadn't decided at the time how I'd finish it. I just took a length of paracord, made a loop and lanyard knot to wrist size, then pulled that through the TH knot, and ran a little bit of needle/thread through each end to keep it in place over the paracord.