M1: Install via ZXP Installer (Recommended)
S1:

Close Adobe applications. Quit Photoshop and/or Illustrator if they are currently running. Installing while the host app is open may cause the installation to fail.

S2:

Enable developer mode for CEP extensions (one-time setup). SciSuite plugins are self-signed, so Adobe requires developer mode to be enabled before they can run.

Windows (quick): Download EnableCEPDevMode.zip, extract it, double-click the .reg file inside, and click "Yes" when prompted.

Windows (manual): Press Win+R, type regedit and press Enter. Navigate to HKEY_CURRENT_USER\SOFTWARE\Adobe\CSXS.11 (create the key if it doesn't exist), create a new String Value named PlayerDebugMode and set its value to 1. Repeat for CSXS.9, CSXS.10, and CSXS.12 to cover all versions.

macOS: Open Terminal (Finder > Applications > Utilities > Terminal), paste the following command and press Enter: for v in 9 10 11 12; do defaults write com.adobe.CSXS.$v PlayerDebugMode 1; done — then log out and log back in (or restart your Mac) for the changes to take effect.

S3:

Download & install a ZXP installer. Any of the following will work: Anastasiy's Extension Manager (recommended — supports drag-and-drop, managing, and uninstalling extensions), aescripts ZXP/UXP Installer (free, lightweight, supports drag-and-drop on macOS and double-click on Windows), or ZXPInstaller (supports drag-and-drop, but does not support managing or uninstalling extensions).

S4:

Load the .zxp file into the installer. Drag the .zxp file into the installer window, or use the File menu to browse and select it (depending on which installer you chose).

S5:

Launch Photoshop or Illustrator. Open the host application. The plugin should now appear under Window > Extensions. If it does not appear, try restarting your computer. On macOS, you may also need to allow the extension in System Settings > Privacy & Security.

M2: Manual Installation (extract and copy files)
S1:

Enable developer mode if you haven't already (see Method 1, Step 2 above).

S2:

Rename and extract. Change the .zxp file extension to .zip, then extract it with any archive tool. Make sure the extracted folder contains manifest.xml directly inside it (not nested in an extra subfolder).

S3:

Copy the folder to the extensions directory.

Windows: Copy the extracted folder to either of the following paths:
C:\Program Files (x86)\Common Files\Adobe\CEP\extensions\ (system-level, may require administrator permission)
C:\Users\<your username>\AppData\Roaming\Adobe\CEP\extensions\ (user-level, no admin needed; create the folders if they don't exist)

macOS: Copy the extracted folder to either of the following paths:
/Library/Application Support/Adobe/CEP/extensions/ (system-level, may require admin password)
~/Library/Application Support/Adobe/CEP/extensions/ (user-level, no admin needed; create the folders if they don't exist. To access ~/Library in Finder, press Shift+Command+G and paste the path)

S4:

Launch Photoshop or Illustrator. The plugin should now appear under Window > Extensions.