Q How do I enable transparent compression?
You don't. You can't. But keep reading.
This is not "enabled" – for the whole filesystem – and then constantly applied, for example for newly created files.
Using filesystem compression features
What you can do is compress transparently individual files and folders.
(One option for this would be using ditto or like in jksoegaard's answer, afsctool, alternative fork)
This distinction is important as it sets limits on what you can compress, effectively.
What to compress, and what not
You can compress your entire home-folder with this.
But that makes not much sense.
In part because many file types are already compressed. JPG, PNG, PDF, zip etc are all formats that usually do have some compression. Compressing them again doesn't save much space but adds a redundant de-compression penalty on accessing them.
In part because how Apple implements its filesystems. Files do not get created and then added to if you amend the content. They get newly created and the old version destroyed. Newly created files don't get and constantly changing files lose all benefit of transparent compression the next time they are written to/committed to the filesystem. This includes all temp-files, cache files, log-files, many preferences etc.
Thus, you should only apply this to files that do not or rarely change.
Those that are read but not written. Application files are the prime candidate for this. And often a great candidate for space saving, as some developers have really atrocious packaging and deployment habits of waste and bloat included.
Tools to use
Apart from afsctool and ditto, GUI tools to achieve this would be MoreSpace Folder Compression (seems to discontinued) or Clusters (v1.7.2, 13$, last updated 2014).
Precautions for unexpected comnequences
Further, you have to be a bit careful in using this.
Unexpected results are very rare, but exist.
For example, a binary of
grep seizes to function and compressing
git results in your git-repos becoming unreadable unless you revert that compression (the repositories are still fine).
Some GUI applications do perform some homebrewed tamper detection that might fail if they are compressed in this way. You might never run into one of those, but it's still best to have a backup ready and proceed in smaller steps, not by compressing all at once.