A cap is used to block DC, a wire is used to connect to the amplifier input.
Shielded cable, 1 end gets shielding grounded at 1/4'' phono plug sleeve lug [cut a cable in half, lol..have 2 audio probes or...]
The tip connects to the signal wire in the center of the cable.
at the cut end of the cable, strip back the shielding say 1'', the signal wire to 1/2'', adjust as necessary to get it looking good, separate wires and all..
Twist the ends and put a DC blocking capacitor, solder a nonpolarized capacitor to the signal wire [rated higher voltage than the circuit input power will ever see].
Plug in the unadulterated 1/4'' phone end, attach the shielding to ground of the circuit to be probed, probe for AC/signal through the capacitor'd signal wire.
That's it, cut a cable, DC block the input, ground the shielding.
A little stretch of flexible ground wire, and insulating the ground [electrical tape] make it easier/safer to use.