I do it one of two ways. Sometimes I double it over, creasing along the centerline, then cut to width. I do that mostly in larger scale, like 1:32. For smaller scale I often just cut a stripe out of the tape (tape pressed against a cutting mat. The single layer then has adhesive back to stick it to seat. For small scale I use PE fittings. For 1:32 I often bend fine craft wire into fittings.
Also, for 1:32 (the double layer method) I run the strip through the buckle and double back upon itself to create the "adjustable" section. For the smaller scale I usually dispense with the adjustment piece.
Masking tape has a color very close to one of the colors of the old military belts.