Upload the productvariants.ocmod.zip file from Extensions > Installer. Refresh the Modifications from Extensions > Modifications. Then Go to Extensions > Modules.You will see the Product Variants module.Click on Intall. After successful install You can configure the settings as per your requirements.

Extract the zip file.open the 2.3x/2.x folder.Upload admin,catalog folder to your server from the "Upload" folder.
Upload the install.ocmod.xml file from Extension Installer. Refresh the Modifications from Extensions > Modifications. Then Go to Extensions > Modules.You will see the Product Variants module.Click on Intall. After successful install You can configure the settings as per your requirements.

Enable the module status. Choose the option accoding to your needs. Choose product fields to display on product variants. Choose autosuggest products as variants. Add Custom CSS.