You'll want to use as sharp a tool as you can find - probably a large kitchen knife - and large BBQ tongs to hold the cactus, and probably heavy gloves also. If it's a cactus with several arms (you don't say what kind of cactus it is,) you might want to simply cut off the tallest arm at the soil level. If it's a segmented type, you'd want to cut at the segments to maintain the attractiveness of the plant. If there is no alternative, you'll have to cut in the middle of section. As far as the cactus is concerned, it doesn't much matter where you cut it. Your concern should be for the aesthetics.
Of course, after you've cut the piece or pieces off, you can easily plant them to make more cacti.
This article has more information: https://www.gardeningknowhow.com/ornamental/cacti-succulents/scgen/propagating-cacti-and-succulents.htm
