Help converting php/css/js pages for WordPress

Hi

I have been helping a friend update their website and they would like the new site set up in WordPress so they can update the text in-house. I have manage to get most of the site set up but I have one particular page that displays a flowchart created out of a php file, a css file and a js file that I would like to copy across to WordPress but I cannot work out the best way to do this.

Presumably I don’t want to set it up as a new theme as this will then overwrite the general theme for the entire site. I have tried copying and pasting the php file into the html tab etc but this doesn’t work.

Can anyone tell me the best way to go about this please? I am happy to upload the code I have if it helps people to see it?

Would really appreciate any assistance that can be offered.

Many thanks

Jo