Step 1: Crochet dog harness
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.)
Hardware (buckle, clip, clasp, etc.)
Optional (measuring tape, lighter, yarn or embroidery needle)
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.
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.)