This should help you to get started with it: https://docs.npmjs.com/getting-started/installing-node
Here is Node.js: https://nodejs.org/en/
After that download the zip from github and unzip it in your downloads folder.
Now you should have a folder named "textexpander-to-alfred3-master" in your downloads folder.
Open Terminal → Type "cd Downloads" and press "Enter" to get into your downloads folder → then "cd textexpander-to-alfred3-master" press "Enter"
to get into the folder of the textexpander converter.
After that type "sudo npm install -g textexpander-to-alfred3" → you will have to type in your password (it won't show any signs or stars, so don't wonder)
Type in your password and press enter. It should run your command and give you something like this:
/usr/local/bin/textexpander-to-alfred3 -> /usr/local/lib/node_modules/textexpander-to-alfred3/textexpander-to-alfred3.js
/usr/local/bin/te2a3 -> /usr/local/lib/node_modules/textexpander-to-alfred3/textexpander-to-alfred3.js
After that you're almost done. Export groups from TextExpander and copy pasta them into the folder "textexpander-to-alfred3-master".
As you're still in that folder via Terminal you just have to type in something like "textexpander-to-alfred3 Signatures.textexpander" depending on your exported filename. It will create a file for Alfred that you can double click and you're done