I’ve done it at Chiang Mai lots of times. Get your document from the helpers, near front doors immigration. They have a copy house at the back of the building to copy passport etc. They can also take your photo there. Fill in your form while you wait. Whole process takes approx 1 hour. They close between 12:00-13:00 for lunch. It’s best to go when it opens in the morning or, at 13:00, when it reopens after lunch.
You can bring it. You can smoke it in your hotel, no issues. You will see people smoking them everywhere, including in bars. Just ask staff if it’s ok before you do. And don’t blatantly use it in front of police.