Simples. For mast mount footage get some off cut or spare kite rope/string, tie it to the Gopro housing and again to the small rope loop on the top of your sail.
For boom mount same thing just less length. Its all very easy.
For board mount on the nose (flat disc supplied in package) trace around plastic mount, sand and stick down. Again simple.
Lash out and buy one. You've got the money