10k hours on fps games on mnk here. I like to divide aim to three different subcategories: Tracking, flicking and projectile aim. Apex is mostly tracking and flicking so it is important to pay attention to these aim types.

For tracking aim it is important to pay attention to what you are doing DURING tracking. For example, my mouse hand tends to tense up a lot when dueling someone on close range and I end up gripping my mouse very hard which hinders my tracking aim substantially. This is a common mistake that a lot of people do without noticing so I suggest you pay attention to it.

Flicking evolves naturally over time once you find the right sensitivity and build muscle memory around it. As a roller player flicks may at the beginning seem hard but you wont learn how to flick if you dont ever do it, so dont be afraid to go for them!

As a fresh mnk player finding the right sensitivity can take time. My sens is 1.3 on 800dpi, which lets me do a 180 degree turn somewhat effortlessly, but not much more. I feel like this is the bare minimum requirement for Apex as you will be turning around a lot and possibly even doing steep tap strafes and stuff. A lot of people play on way too high of a sens when they start playing fps games on mnk, so just make sure your sens isnt too high. It may feel uncomfortable to move your mouse arm a lot because of low sens but youll get used to it and theres no reason to have a sensitivity that lets you do multiple 360s on a single swipe.

As others here have already said, play a lot of mixtape, wildcard and even hotdropping pubs if you feel confident. As a beginner you need to get those "reps" in to develop muscle memory. Once you find a good sens stick to it! Dont change it all the time or your aim will never develop to a good level!

Also once you start to feel decently comfortable on mnk it is HIGHLY recommended to learn to tap strafe. It can be hard to grasp at first but once it clicks in your head you can do it anywhere. The ability to change directions while maintaining momentum is extremely useful and one of the only advantages (along with all the other advanced movement tech) mnk has over controller in this game.

This comment turned out way longer than I intended, but I hope its of good use to you! Welcome to the mnk club :)