When

It depends on your objectives.  The physical benefits of a sauna bath occur regardless of the time of day. 

The mental benefits can vary.  Sauna sessions in the morning can help clear the mind and put you in a positive mood for the day.  Sauna sessions during the day can help interrupt the stress pattern of the days activities and enable a reset as you move into the rest of the day.  Sauna sessions in the evening can be a great ritual to prepare for a restful night of sleep. 

The best time to use the sauna, is the time that your schedule affords it to be a regular routine for you. 

How

If you are new to sauna baths, target 30 minutes with heat at 110 - 130 for your first 3 sessions. This lets the body gently become familiar with the experience of a sauna bath. 

From that point, pay attention to your body and mind as you determine you favorite sauna setting (time and temperature) within a max time of 45 minutes and a max temperature of 150 degrees.  Therapy is optimized when you have induced sweating and then remain in the sauna for 20 minutes more.  The more you sauna, the faster your body will cooperate with the sauna to induce sweating.

Disrobe as much as you are comfortable with.  The infrared provided by the sauna panels is beneficial for your skin and is the main component of this therapy, so let as much of your skin participate as possible.

Towels are your friend.  A towel on the bench protects the sauna and maintain sanitation from one session to the next.  Your body towel should be used to wipe sweat several times during your session as this will cause your body to sweat even more to replace the moisture.

After your sauna, wipe down thoroughly with your towel.  Give your body 15 minutes to continue processing the therapy you just provided it and to adjust to normal temperatures.  If you wish to shower, avoid a hot shower.  Room temperature water or cold water can provide a contrasting experience for your skin and body that is therapeutic as well.

Hydrate 24X7 and during the session.  But be careful to not cause an interruption to your session by having to visit the restroom during your session time.

Summary

Find a time in your schedule when you can sauna regularly

Find a temperature and time that let you induce a sweat and then have 20 minutes remaining in the sauna

Expose as much skin as you can, and wipe off the sweat a few times during your session

Thoroughly wipe yourself off after your sauna.  Optionally shower 15 minutes later with room temperature or cold water.

Hydrate all the time and during your session, but avoid restroom breaks that interrupt your session.