The official instructions are here. They seem to work well, although when you clone the question behaviours in step 3 I think you’ll need to do something to make the permissions correct (in Debian everything needs to be visible to the www-data user). Prefixing the git commands with sudo -u www-data is one way to do this.

The default version of Maxima in Debian 9 is 5.38.1, which STACK seems happy with.

The quick start guide for authoring questions is a good next step.