First steps
Decide whether the new person(s) and everyone else will benefit from them being a member of the Telegram Pirate Care Group, our main communication channel.
If yes: provide them with a link to the Pirate Care Group and/or ask them to download and sign up to Telegram.
Get them in touch with @marcell and @toby on Telegram who can help set up a Pirate Care Syllabus git account for them in Gitea, the development infrastructure that supports Syllabus, which you are using now :)
If it has already been decided which page(s) should be translated someone (most of the time that would be @marcell or @toby) should add a new session markdown file with the name: LL.SSSSS.md (LL = language code, SSSS = file name of the original session in English) to Gitea.
The new session file should have a good front matter/header which should loook like:
---
title: "Klingon: The original session title"
images: ["the same if it was present in the original one"]
---
link to the original session in this form: 
Send the link to the new person(s) in order for them to start translating....
For those not too (or not at all) familiar with writing text in Markdown, it might also help to provide them with a short primer of how the syntax works, see e.g. Online Cheat Sheet, or neat print-out sheet.
More information on how to edit the files in Syllabus for those unfamiliar with Gitea can be found in the 📁 README document (which is also shown and readable if you go strating to the Syllabus Gitea Repository