Im not a hairdresser so this is all my personal opinion but one tip I can give you is to make sure the line is straight and that you really commit to the style. My bff got her dad to give her an undercut years ago and instead of parting it straight she parted it kinda curved? And I could never tell this to her face but it looked terrible... If you're gonna keep long or long
ish hair on the top then you need to make sure the undercut stands out and is bold, otherwise you're just going to look like you're balding or something. An undercut is a very bold look for someone with long hair, so you need to commit or it's not gonna work.
What my bff had was something like this except it was a lot more curved and it was only like an inch and a half thick at most. Personally I feel like even the girl in the photo looks pretty bad, and that's
with the fact that she has a lot more cut off than what my bff had and has her hair in a very stylish messy bun to give her more style points.
In this pic, even if we ignore the pattern we can see her line isn't curved (tho a bit uneven I must say) and she had a lot more cut off, she's committed a lot more to the bold style and she's pulled it off a lot better in my opinion.
If you didn't want to cut so much off for example maybe you're worried about your hair being thin then I recommend you cut a triangle instead. This girl has a lot less cut off but since it's still a bold shape she still committed to the undercut and it paid off.
For actually parting the hair I can only recommend you have a family member help you if you try to do it urself even with a mirror both in front of you and behind you're still gonna need to turn ur neck slightly to see what you're doing and when you straighten up again, it isn't going to be as straight as you thought it was. Even if ur family sucks at doing it for you, you're still better off with them doing it for you. That way you can get them to take a picture of it while you're standing straight so you can see how it'll actually look, and you can always instruct them how to do it better if the picture doesn't look like how you wanted it. To actually shave it off you might be better off putting a headband over the part that you dont want shaved just so you don't accidentally shave it off after you spent so long parting it perfectly, i'd also recommend you just get someone to do this for you~
I'm sorry if it felt like I talked down to you in this entire reply or something I just don't know how much you've already researched about it so it's better to talk to you as if you didn't know anything, although i'm sure you're well educated about this if you're considering doing it~