So, a paracord holster costs around £7... Need I say more?
Anyway, I decided to make my own. They are (very) easy to make, are relatively inexpensive, and look great.

Step 1: Stuff You'll Need

Paracord.
Carabiner.
Cord lock.
Scissors.
PVA glue or lighter

Instead of a paracord you can use the 'handles' of a drawstring gym bag, or shoelaces.
And instead of a carabiner, a key ring would work too.
Furthermore, instead of a cord lock, you can use a large bead, however, those do not hold the whole thing very well. (For one of my holsters (see blue one in images), I just use some more paracord, to make a slider, which holds it pretty well.)

Step 2: Step 1

Cut the paracord into 2.

(Starting off nice and simple)

Step 3: Step 2

Thread the two pieces through your carabiner. The two ones at the front will be in the middle, and the two at the back will be knotted together to make the zigzaggy pattern.

Step 4: Step 3

Start pleating, or whatever its called... Remember to do both sides - so at first do what the image does, then do the same thing, but opposite.
Does that even make sense?
It is difficult to explain, but if you do it wrong, you will notice, as all the 'lines' will be going in the same direction.

Step 5: Step 4

Do this for about 10cm or until just over half of your yoyo can be covered with it.

Step 6: Step 5

Now, take out the carabiner.
You should see two small loops. Thread the ends of the two middle cords through these - one in each loop, ensuring they don't cross or overlap.

Step 7: Step 6

Next, slide the cord lock onto the two middle cords.
You may place your yoyo in, and see how the holster will look. If you are not happy with the 'woven' bit, you can make it longer or shorter.
Likewise, you will want to make sure you have a good length of paracord left. Around 200mm should suffice, but will depend on the size of your yoyo. I made mine big enough for my largest yoyo to fit in.
If in doubt, make it bigger, rather than smaller.

Step 8: Step 6

Slide the cord lock down. Check your yoyo's circumference against the middle cord's lenght, and cut the two cords 8-13cm (again, depending on yoyo size) from the cord lock, and tie a knot. Do one final check, to ensure the yoyo fits.

Step 9: Nearly There

Cut the two side cords (the ones you weaved), relatively close to the weaved part.
Now, put a splodge of PVA glue on the ends of them, and the end of the single knot.
An even better way to 'close' them, is to melt the ends with a lighter- which is what i did. This way it looks much neater, and makes it waterproof, but it can disrupt the colour pattern, and one would have to be careful.

Step 10: Nearly...

Now, just slide the carabiner back into the two loops where the middle cords are. This might be tricky, dues to tightness.

Step 11: Done!

Now you can slide the yoyo into the created loop, and push the cord lock down, clip it on your bag, an you're all set!

Hope you got it. Would love to see how yours turned out!