How do I tighten this properly?

You only need to tighten the inner 8mm hex. It should (if it’s a decent rip-off of the DUB style but with a 24mm spindle) go up to ~54Nm which is a massive amount of torque. Like, you need a decent sized wrench + 8mm bit to do it.

You need to (again, assuming DUB style due to the use of the preload collar visible in your second photo)

– Tighten 8mm hex to 54Nm

– set preload by twisting collar to remove side-to-side movement

– tighten the tiny hex (very lightly probs like 1-2Nm) on the collar

That massive outer hex that’s visible in your first photo is just the extractor cap, since your cranks are self-extracting.

https://preview.redd.it/w9ql4cfsonbe1.png?width=379&format=png&auto=webp&s=9e437084612d7856e6542152560f578a82209d7f

Here is a screenshot from one of the product pages.
You need to tighten it to 54Nm, that is A LOT. A lot more than you can do with a “normal” allen key. You need a LONG wrench, and even then you need to put a lot of force (your whole body weight) on it.

For reference, a cassette needs to be tightened to about 40Nm, so this is even more than that.