Any advice on drilling holes in a container to use for a balcony garden?

The placement you laid out looks good but without knowing the size of the pot, it’s hard to tell what size bit. Maybe a 1/2”-3/4” bit? And mayyybe adding another 2 holes.

One thing I would recommend though is to use a small bit to start your hole, then move up to the bigger bit. Don’t put too much pressure on the drill so you don’t crack the plastic. Let the drill and bit do the work.

It also depends on what you're going to put in there, and in what sort of soil.

These holes are generally good. For example a succulent/cactus and others which prefer good drainage would like them, as long as the soil mix was light and had good drainage.

Then again, if you're looking to grow tomatoes or something which might get really thirsty, you might actually consider drilling a hole(s) in the side of the pot an inch off the bottom or so. This is so that when you water it, you water until it comes out from the side hole, so theres always a bit of a "water reservoir" at the bottom.

But that won't do with plants which actually don't drink so much, as it'd just make it more likely for their roots to rot.

The technique as used in that form but by weed growers, is called "a hempy bucket", and you fill it with a light, well draining soil mix. Or no soil at all is pretty common (as in you still have a growing medium, but it's not soil, as in dirt) coconut husk/vermiculite mixed with perlite in about a 5050 ratio will make it "passive hydroponics".

Here's a guide. Don't worry even if it's for growing weed, the same applies to tomatoes, chilies and potatoes and whatnot harvest plants you might be growing on your balcony. https://www.rollitup.org/t/an-idiots-guide-to-hempy.386008/

But as I said, the bottom holes are fine. Just thought to share some info in case it's useful.

Edit forgot the link

Keep in mind how the pot is going to sit. If you are setting it on a flat surface, the holes need to be able to drain. I like having holes on the very bottom as well as in elevated sections. If possible, set it on blocks or something. Also, if you bottom water, its nice to have holes that aren't the very bottom

Don't overthink it. I would try a few different kinds on different pots and then decide from there. Personally I used to be very worried about this, I used to put debris like rocks in the bottom, etc but over time I have found that when growing things in container I generally like the least amount of drainage possible. I do just enough that the plants cant drown. This saves me on watering excessively. Particularly on my roof garden, it was getting so hot, I just struggled to irrigate adequately and would use a lot of water. Now I just up-potted like a half dozen nightshades for my garden (its raised beds now but theyll be in containers until frost is not a threat) in upcycled yoghurt containers. I cut very thin slits into the containers haphazardly with a rusty old pruner. They'll do fine. I guess it would be different if you live somewhere super rainy, but somehow I doubt in container garden you will be worried about perfect drainage.