Which side of the fastener a spring washer is put?

Spring washers can be used on either side of a fastener (nut or bolt head), though typically it would be installed between a fastened surface the nut.

Spring washers provide tension between the threads and the washer surface against the mating piece when compressed, acting as a lock washer which prevents nuts and bolts from loosening as a result of vibration that the joined items may be subject to.