Attach a short piece of 1" strap at the top of the sheath and another low on the sheath, around the sheath as a loop as if you would then put it horizontal on a belt.
Kydex pieces will work better if you can make them or obtain the right size somewhere.
Tek-Loks will work, but I don't like them for this use.
Then run a piece of 1" strap through these loops in line with the sheath. Use some tri-glides/ladderlocks to attach these to the shoulder strap you'll use to carry the Waki and sheath.
Make the attachment to the shoulder strap as tight as possible.
Add a stabilizing strap around the waist or chest maybe.
You can use any size strapping you want, but I use 1".
For Kydex for something like leatherman suggested up the handle, a SKDL from Rainwalker would work well or make something similar but longer yourself out of a single piece of kydex with holes drilled at the right spots.