Choose the center speaker location

Measure the distance from the listening 'sweet spot' position to the front left or right speaker.  This measurement is the distance from the 'sweet spot' to the center speaker location centered between the front and right speakers.

Select rear speaker locations

Dolby Labs 5.1 surround sound specifications position the two rear channel speakers at 110 degrees left and right of the front center speaker. 
Draw a circle through the front left, center, and right speaker positions with the  'sweet spot' at the center. 

Place the left and right rear speaker on the circle at the specified 110-degree positions. 
Provide unobstructed sound paths to the listener's ears.  This may require moving the speaker above ear level.

A 6.1 and 7.1 surround system adds additional rear speakers to the 5.1 arrangement.  A 6.1 system places an additional-rear speaker on the circle at 180 degrees from the center speaker.   A 7.1 system adds two rear speakers placed at 150 degrees left and right of the front center speaker.

Note for the smaller room and rear speakers.

If room boundaries are closer than specified for rear speaker locations, move the speakers well above or below ear level.  Or consider the option of an above ear level on-wall dipole/bipole speakers placed at 90 degrees left and right of the front center speaker.  Each option is a compromise.  But this can generate a more diffuse sound pattern that stimulates the desired effect.

Subwoofer Placement

There are two general-schools regarding subwoofer type and placement.  The first school places the subwoofer at a position that avoids stimulating a primary room mode and its first, second, and possibly third harmonic, which locates the subwoofer in an area less than 25% but more than 10% from the dimension of any room boundary.

The second school places the subwoofer in an acoustical pressure zone, the corner of a room.  This type of placement acoustically stimulates and endeavors to manage and use low-frequency room modes.  Follow the manufacturer's placement/adjustment instructions for the best results.

An alternative to the first school temporarily places the subwoofer at the listening 'sweet spot' position.  Set up a quick temporary adjustment of the subwoofer calibration.  Then while listening to a musical recording with good bass content --- walk around the room.  Locate positions where the subwoofer sounds best. 
Select a practical option.  Then move the subwoofer to that position.  This position should reciprocate the acoustical-effect of the listening position. 

Another option --  Use two or more subwoofers.  Two subs more easily raise decibel level and improve dynamics. 
Subwoofers placed opposite each other may also offset room mode issues.

Subwoofer Calibration

Finally, with each of the above methods, set the subwoofer volume to its minimum level, and its crossover to its highest position/frequency.
Next, using white-noise as a source and a C-weighted SPL meter, adjust each of the main speakers to a reference level of 85 decibels. 
Then adjust the subwoofer to the same decibel level.  Now, using a musical source -- adjust the subwoofer cross-over, fine-tune its volume level,
until the subwoofer sounds musically balanced with the main speaker system.