Start by cutting off about a 12 inch piece of 1 pair cross connect wire or some other type of phone wire.  The length isn’t really important, just give yourself enough to work with.

Next you will need an RJ45 crimp on plug and a RJ45 crimping tool.

The pin-out for a T1 loop back is pin 1 to pin 4 and pin 2 to pin 5.  Take one side of your wire and insert it into pin 1 on the RJ45 plug. Bend that name wire over and insert the other end into pin 4.  Now take the second pair of you wire and insert one end into pin 2.  Bend this wire over and insert the other end into pin 5.  Give this plug a good crimp with your RJ45 crimping tool.