Download and install Fiji. Afterwards, click the menu Help > Update... and activate the [CLIJ update site](installationInFiji.
CLIJ is successfully installed, if you find a menu entry Plugins > ImageJ on GPU (CLIJ).
On Windows and MacOS installation works right away in case the computer has a build-in Intel HD GPU or AMD Ryzen GPU. In case dedicated AMD or NVidia GPUs are used, special driver might have to be installed. Search for more information about drives on the website of the GPU vendors.
Linux users need to install drivers for OpenCL, even on Intel HD GPUs. It is recommended to install the packaged beignet and ocl-icd-devel.