Many of our instruments have a 1/4 input that will accept guitar level signals. The most immediate and direct way to use a guitar or other string instrument (that has a pickup) with a synthesizer, is to plug it into this EXTERNAL INPUT jack, to take advantage of the built in ladder filter found on most of our instruments. Keep in mind, that in this scenario, you are using the synth like a guitar pedal or studio effects processor to do things like roll off the high frequency content, add resonant feedback, or achieve tremolo like movement.*
Instruments like the Grandmother and Matriarch have built in reverb and delay effects that you can also tap into with your guitar. Both instruments feature a front panel latch or “drone” switch that allows you to easily open the gate to take advantage of these analog effects without having to touch the keyboard. The switch would be flipped to the rightmost position to open the GATE (set to DRONE) and allow the guitar’s signal to pass through.
It is also possible to use a guitar to play notes on a synthesizer, in lieu of a keyboard controller. To do this you need a guitar to MIDI converter box, which is widely available online. Some are more sensitive and track better than others. Low notes are typically a little harder to track but it is possible. Some artists who feel more comfortable playing guitar or bass than a keyboard, have been doing this with much success for decades. One such artist is Squarepusher, who is a virtuoso bassist and electronic music producer. Most of the synth lines on his records were actually created using his bass guitar as a controller.
*If your instrument does not have a line level output, you might try a preamp to boost the signal before you process it with your Moog Synthesizer.