Steps:

1.) Make sure you are quite far underground

2.) I suggest making a 3x3 tunnel leading to where you are making the hot tub

3.) At the end of your tunnel place one block of TNT to form a rough square room.
(you can place more than one TNT block depending on how big you want it)

4.) Clean out the room by getting rid of excess block in the way
(Try and make 4 walls completely straight and a flat ground)

5.) Make one layer of wooden blocks going around all 4 sides of the room
(or any other block you like to use as walls)

6.) Make a stairs 2 blocks wide going down to the bottom of the hot tub.
(I suggest making the actual hot tub about 5-6 blocks deep)

7.) Add another layer of blocks one cube out from the walls going down to the
bottom of the hot tub which will be where players can sit.

8.) Add another layer of blocks one cube out and 2 cubes lower than the seat going
all the way down to the floor.

9.) Add deep blue lights around the hot tub or build in to the walls and put a block of water in each corner of the hot tub to make a nice touch.

10.) DONE.


Thanks