During my Sauna build I couln't really find good instructions and tips, so I'll try to explain few details I learned along the way. Hope this will be useful for you.
Here are plans of the benches for inspiration https://drive.google.com/drive/folders/1WmRkTCKC2NTJ1hRjOvQflW_cEGimL09k?usp=sharing (just do it out of smaller balk, mine was way to massive)
Paraffin oil for benches - https://amzn.to/40TOkCJ (don't worry that it's "for lamps", same stuff)
Tongue and groove clips - https://amzn.to/3E8IPp9
0:00 Intro
0:49 Ventilation
3:10 Wall construction
6:34 Insulation
11:45 Wood paneling
17:48 Heater
20:39 Window/Door
22:23 Benches
28:18 Light
30:30 Dimensions
33:47 Outro
All music in video is by Dan Lebowitz