Getting BasicMod into an IDE
I strongly recommend using IntelliJ over Eclipse. Having used both, I've found IntelliJ much easier to use/get started with. In addition, almost all the modders in the official Slay the Spire Discord use it and will be more easily able to help you if you do as well. As such, this tutorial will be explaining how to setup with IntelliJ.
If you have your own preferred IDE, you'll have to figure out how to import a Maven project into that IDE.
You can find the free community edition here (scroll down a bit, past the Not-Community edition). Installation instructions are also on that page. You (probably) don't need any of the features provided by the Ultimate (paid) edition.
Do not clone or fork unless you really want to go through the hassle of changing git things later if you want to upload it to your own separate repository. If you do know what you're doing, you can use this repository as a template.
Unzip wherever you want to keep all your modding files. I highly recommend creating a dedicated folder for modding, as well as an individual folder within it for each mod you create.
When you first open IntelliJ (after setup) you should be greeted with a screen like this. The color scheme will depend on what you chose during setup.
Choose open, and then find the file pom.xml wherever you unzipped BasicMod.
Open it. Be sure to choose the pom.xml file to ensure it opens correctly. This will give you a popup saying that pom.xml is a project file. Would you like to open the project?
Choose Open as Project. It may also ask if you trust the project. Hopefully you do.
After a brief period of importing, you should have something like this. The README may or may not be open.
(Note: IntelliJ has released a new UI update, so things may look different. However, the general placement of things is mostly the same.)
If so, you can move onto the next step.