As long as you are talking about using an ancillary speaker cabinet as a stage monitor for just your GUITAR amp output, (and not the total band mix including vocals) then it's quite easy;
First, you examine the back of your guitar amp to see if it has a 1/4 jack output for sending the guitar signal to a second speaker. Many amplifiers offer this feature, and the output jack lists the wattage and ohms requirements for that other speaker. It may be labeled OUT, LINE OUT, or EXT SPEAKER.
It's not recommended to use the EFFECTS OUT jack for this purpose, because the output signal is quite low, and plugging a cable into that jack may defeat the amp's main speaker output altogether (anticipating that there will be an EFFECTS IN plugged in in order to complete the loop).
Use a standard guitar cable to chain your amp to that speaker, and take the time to check your stage volume versus monitor volume. Don't forget that the drummer (if you have one) needs to hear your guitar sounds just as much as you do, so speaker placement is paramount.
I have seen players chain to a small ancillary speaker by using standard two-strand speaker wire and two pairs of alligator clips, and connecting the amp speaker tabs (+ and -) to the 2nd speaker's tabs. This is sort of a cheesy solution, but it certainly gets the job done, depending upon your stage requirements and traffic.