Please pay attention on how these icons are licensed and make sure to respect the license type. Most of them, including IconFinder’s free icons, require attribution.
You should notice the Icon Information icon in each icon’s toolbar in My Collection page, eg: https://d.pr/i/JljeUc . Clicking on it will open a panel with informations and a premade attribution HTML code. You can Copy it and paste the code where you want to show the attribution.
You can insert the attribution on the page of the icon (for example in the page footer) or on the imprint page. Or, you can place the attribution on the credits/description page of the application/website.