You don't have to create a database, a database user or a sub-directory to install IWP.
It will use the WordPress's database by default, with an iwp_ prefix.

You can change it by clicking on the "View DB Details" link.
And the sub-folder will be /iwp by default. You can change this too.

Step 1: Download the installer plugin here - Download Installer Plugin.

Step 2: In your WP admin dashboard, click on Plugins → Add New

Step 3: Click on the Upload link.

Step 4: Click on the Choose file button and select the file that you downloaded in Step 1.

Step 5: After the .zip file is uploaded, hit Install Now.

Step 6: After it's installed, click Activate Plugin.

Step 7: Click on the big black Install InfiniteWP admin panel button and follow the on-screen instructions.