How do I edit a bookmarklet to replace slashes with plus signs, but then add back in a new URL at the beginning?

Question

I need the bookmarklet to do this:

1. Replace all slashes with plus signs.
2. Add a new set of text to the beginning, before what was done in step 1.
3. Open the new URL in a new tab.

For example:

1. Turn the present page that's open, say https://example.com/files/other/finally, into https://example.com+files+other+finally.
2. Isolate those items after the primary website name (files+other+finally) and add https://example.com/documents/pages/ to the start, to make it turn into https://example.com/documents/pages/files+other+finally.
3. Open that final URL in a new tab.

I've tried several things like location.href.split("/").pop()) and others. But I'm completely new to this, so it's confusing, and I'm hoping someone can swoop in to make me understand this.