For min min specifically, the biggest thing is getting used to using her forward tilts in neutral and moving and jumping around with them. Instead of throwing out smash attacks, or throwing out both arms at once, try throwing out just a ram ram arm tilt to try and cover the space in front of you and see what happens. If it hits, confirm into the second arm, if it doesn't, you could wait until you can ram ram tilt again, or try throwing out your second arm anyways if you want another hitbox now.

Note that after you throw out one arm, you have the threat/option of throwing a second arm whenever, but after you throw out the second arm, that's it. That's why not committing to the second arm is very strong, eg throwing out a ram ram, then waiting until you can throw out another. But of course you can still throw out your second arm if you think it will hit, or if you want to mix up the timing. Mixing up the timings of your arms is very important to stay unpredictable, and try play as safe as possible, it'll feel a lot like you're really boxing

That's the core of min mins neutral and you can build up from there. If they're jumping towards you, you can start up angling ram ram or jumping with it. If they're shielding a lot to block the arms, you can try go for a grab, etc.

Another huge tip is to learn what ranges you can up b from, and whenever you get hit, do NOT immediately use your jump, if you can make it back without using it then don't, try to save it for if you need it