I looked into it, but haven't done it, likely won't.
It's not a secret, just an involved industrial process.
If you really wanted to do it, these people will sell you the products and give you technical support to make their products work
For knifmaking uses, It's cheaper to buy or order belts than to setup the process.
I can buy cheap AO belts at Pop's for $1.90 each
Using their adhesive coated tape
cut splice
remove the grit from the splice area to keep the belt thickness the same at the joint (sandblast or wire brush)
good alignment
heat pressed fit 180 Degrees F and 2000psi
the adhesive tape has a limited shelf life which means I would throw more away than use.
"ADK 4-NNF is stable for
60 days when stored below 0° (-18°C).
It must be protected from exposure to heat, humidity and direct sunlight.
The shelf life of this product at room temperature is less than 4 hours."
http://www.adktapes.com/products.html
http://www.adktapes.com/ADK_4NNF.html
http://www.adktapes.com/techinfo.html
oh and I found all this info with the Google...