Download and install the latest IDEA version.
Download and install the latest version of the Cursive plugin.
After restarting IDEA, Cursive should be working out of the box. Follow the user guide to fine-tune appearance, keybindings, code style etc.
Cursive is a commercial product, with a 30-day evaluation period. Unlike
IntelliJ, it doesn't have a community edition. Free non-commercial licences are available to individuals for non-commercial use, including personal hacking, open-source and student work. The licence is valid for 6 months and can be renewed.