User em36 from Columbia University offers a full Mac OS 9 solution ready to install. Just:

Et voilà!

Now all you have to do is run Mac OS 9.app The unix drive in it will map to your Documents folder, allowing you to easily swap files between macOS classic and OS X.

The site linked above includes a similar ready-to-use solution for macOS 7.6.1 and 7.5.5.

The same user also offers a ready-to-use solution for Windows, see Mac OS 9 for Windows.

Of course, these solutions assume you trust the files to be safe to use.

Another option is to just install everything yourself. SheepShaver is the de facto emulator for this. To do this, you will need:

You'll have to put your hands on the ROM and the macOS CD or image yourself. Once you do, the relevant links to set things up are:

SheepShaver is not particularly intuitive and quits without an error message when things are not setup like it expects them to be. So, you'll want to pay attention to the instructions, reading them in full and making sure not to skip a step.

Some things I found worth bearing in mind:

A great joy for me as a teenager was the pre-Internet culture of e-zines. You'd buy magazines at your neighborhood newsstand and, on the CD-ROM that came with it, was a small collection of e-zines (most often created with DOCMaker.) I was running my own e-zine at the time and people from all-over France would write to me to contribute content – a fond memory 🙂

Classic Mac e-zines:

Paper magazines that included the e-zines on their cover CDs:

I'll add more links as I find them.

Being able to run old macOS software in an emulator is all fine and dandy... But sometimes you may need to export stuff for use in a modern operating system.

For example, in my case, I had old DOCMaker applications and wanted to export their content.

A simple way to do this for me was to just "print" the document into a PDF.

PrintToPDF, for macOS classic, lets you do just that.