How do I go about transferring 90k altitude points to Thai airways?
I’m afraid you can’t. Altitude points transfer to Velocity, Asia Miles (Cathay), Malaysia Airlines Enrich, Air NZ Airpoints, or Singapore Airlines KrisFlyer.
The workaround is that Thai and Singapore are both Star Alliance airlines. If you transfer your points to KrisFlyer, you’ll be able to use your points to redeem Thai flights (Air NZ is also Star Alliance, but I’d recommend KrisFlyer over Airpoints).