There is a tool specific to the adjustment of the XPLR rear derailleurs called the SRAM Rear Derailleur Chaingap Adjustment Gauge (11.7518.100.001), but you can also use a ruler if you don't have access to one.
The chain gap adjustment for XPLR rear derailleurs is completed with the derailleur shifted inboard to the 2nd largest cog. Install the gauge onto the upper pulley, with the top half of the gauge behind the largest cog. Rotate the b-adjust screw until the appropriate gap is achieved for the cassette you are using - the distance is 11mm for 36T cassettes, and 7-8mm for 44T cassettes.