[SOLVED] how to make custom 404 & remove "next post" link

hi folks. first post- please forgive a newbie.

this is my first wp site, and i have limited coding experience, dating back to HTML from ten years ago!

i am hoping to make two changes to my site, both i suspect are easy to the hardened veteran, but my browsing of this forum and my own attempts have been unsuccessful.

1) i hope to make a custom 404. i see the "not_found.php" file in montezuma editor but that is not apparently what is coding that 404 page. when i get a 404 error with my site it says "apologies. but no results were found for the requested archive, etc etc".

do you know where i find that file or text or whatever to edit it? once i find it, i shoudl be able to figure it out.

2) i am using wp as a shortcut to making my own entire site from scratch, and actually dont want people to be able to flip between posts sequentially. so: i want to remove the "previous post" and "next post" links at the top of each post. i (surprise!) cant figure out how to do this, and it might be much more complicated than editing some text.

any hints?

thanks!! (ps: definitely going to donate. im $200k in debt, but this is worth it)