Answer
Loose, comfortable clothing is ideal. Avoid tight-fitting clothes to allow for movement. Natural fabrics like cotton are recommended. Some classes provide traditional Thai massage outfits. Learning Thai Massage can be approached by first deciding the location, time available, transport and level of activity that suits the trip. Use the options in the question as a starting point, then compare current provider information and recent local conditions before making a booking.
Practical Chiang Mai planning note: Look at the qualifications, safeguarding, facilities, language support and suitability for your own health or family needs. A pleasant online description is not a substitute for checking the current programme and speaking directly with the provider.