installing square nuts in plastics pieces  

I find it easiest to offer them up with a pair of needle nosed pliers (tweezers might do) and push them home with a fingernail or an electrical screwdriver in tight corners.

Cheerio,

Pretty much the same here except I started them with the regular-size slip-joint gas pliers as opposed to needle-nose and finished inserting them with a screwdriver.

I found some to just slip right in, but others needing more 'gentle persuasion' to get in there.

RE: installing square nuts in plastics pieces

I use a set similar to this: https://www.amazon.com/Craftsman-9-45673-Joint-Pliers-3-Piece/dp/B006YV22OQ but it's an old Craftsman Professional set made with better quality than what they have now. The set I have came with a 4th plier that has 75* angled jaws which makes it a little easier on my wrists. These should still work fine.

Use the short jaws, and don't over grip the plier, then use one of the jaws to drive it in. Can't guarantee the part will hold the nut, but this works pretty well for me. The side cutters are great for trimming filament, also your fingernails when they get in the way. 

Does it have to be square nuts?

What I am using more and more often ist this - available in sizes M3, M4, M5, M6 and so on. You only need a soldering iron to put them into the parts (thread inserts):

Best regards, Clemens

RE: dito

Indeed. I use these brass inserts almost all the time now.

I dedicated an old conical tip from my soldering iron and with the right temperature it will just slip nicely in a slightly undersized hole.

Never saw one get loose yet. Highly recommended.

Also making a small hole recess is a lot easier than making a proper cavity for a square nut in my cad software.

Happy printing.

There's a time and place for both.

I've become a fan of the brass too. Especially for smaller screws like M2 - M2.5. The inserts are also nice when the screw is not at the edge of the part or when you don't want a hole in the side of the part. I recently got these from amazon: https://www.amazon.com/gp/product/B08B17VQLD/ref=ppx_yo_dt_b_asin_title_o08_s01?ie=UTF8&psc=1
The little tray they com in was actually a 3d print.

The brass inserts are not foolproof though. If you make the hole too small, then you melt the hell out of the part on insertion, but if it's too wide, then the insert doesn't glue itself in tightly and can pull out. This makes them tricky for designs to be shared, because not all printers are dialed in the same.

Square nut inserts have more tolerance on the hole size, and don't require a soldering iron. so I can see them being the preferred approach for shared items.