To begin, make sure to download the latest version of the Prolost Presets you wish to install. Verify that the version of Lightroom you have is supported by the presets.

This installation process involves copying folders of presets into a specific folder on your system drive. We’ll start by locating the folder we're copying to.

Launch Lightroom.
Open Preferences (on Mac that’s Lightroom → Preferences, on Windows it's Edit → Preferences).
Click on the Presets tab.
Click Show Lightroom Presets Folder...
Within the folder that opens, double-click the Develop Presets folder. This is the folder that you'll copy the preset folders into.
In the folder you unzipped, open the For Lightroom 6 folder. There, select all the folders that start with +L, and drag them into the Develop Presets folder.
You should now have multiple folders beginning with “+L” in your Develop Presets folder.
Return to Lightroom and quit the app.
Now launch Lightroom and go to the Develop module. You should see the Preset Groups in the left panel beginning with the +L prefix.