I'm going to show you how to make a custom sized, personalized, and nearly indestructible dog harness using para-cord and a crochet hook.

Step 1: Crochet Dog Harness

Gather materials
Dog (or cat, or, whatever pet you need to put a leash on)
Para-cord / Nylon twine (I'd suggest against cotton, or, hemp, because they tend to rot.)
Crochet hook
Hardware (buckle, clip, clasp, etc.)
Optional (measuring tape, lighter, yarn or embroidery needle)

Step 2:

First you're going to need to know what size to make the harness.
For this you use a measuring tape if you have one, or the way I did it was to knot a piece or the twine then wrap it around the dogs neck with very little slack and knot it again where the two ends meet, repeat this for the dogs chest.

Step 3:

Now we can crochet.
1)Begin a free chain the length of your first measurement. When it is long enough prepare to attach the first half of the clip.
2) Take your loop and feed it through the notches in the buckle, then complete stitch.
3) Now do another stitch going up the length of the buckle.
4)Now we can double over on our chain until we reach the other end, where we'll have to attach the other half of the clip the same way.

Continue this process until about half of the space available for attachment on either side of the buckle it taken up.

When this is finished tie off the first strand and return to part 1 of this step this time to your second measurement continue until you've reached the first half of the harness.

To attach these straps use you're yarn needle and sew them together beginning at the buckle go in 1/8 of the over all length of the shorter strap, repeat on the other side.

P.S.(There is a helpful intro to crocheting here.)

Step 4:

For the next step we return to our pet and put the harness on them, between their two front legs we measure the space between the center of our straps this will be the length of our final piece. Remove the harness from pet and begin your middle piece the same way as the prior two.

1)When you're at the proper length attach your chain 3 stitches to the right of the center of your first strap.
2) Then double through going to the left and add the next row to your chain. At the middle of strap 2 repeat part 1 of this step only this time it should be 2 stitches right of center. continue to crochet until you have 7 good rows (More for a larger animal)

Step 5:

To finish up this project sew on or attach you dog's tags and put the harness on it. If there is too much slack you can bring it in by sewing straps A and B together closer to the shoulder of the dog.