To install the BookIt plugin you need to download the file from the wordpress.org.
BookIt is a free plugin that has a Pro version you can get for an additional price.
Download the plugin archive file here https://wordpress.org/plugins/bookit/ and follow to Plugins > Add New from your dashboard.
Click on the Upload Plugin button and select the BookIt file. Click Install Now and wait for the installation to finish.
After the installation, you need to activate the plugin and you can start working.
Also, you can install the plugin right from your dashboard.
Follow Plugins > Add new, use the search panel, and search for the BookIt Calendar plugin.
Click Install and after installation Activate the plugin.
To install the Pro BookIt Calendar plugin you need to get a paid version of the plugin here: https://stylemixthemes.com/wordpress-appointment-plugin/
After purchasing the plugin you will get an email from Freemius with the link to the plugin zip package, your license code, and instruction on how to set up BookIt Calendar.
Once you get the package, follow to Plugins > Add new and upload the zip file. Click Install Now and wait for the installation to finish.
When the plugin is installed you need to Activate it.
After clicking on Activate Plugin, you will be redirected to another page to activate your license. Use the License Key you received with your email and enter it in the field provided.
Click Agree & Activate License to confirm the activation.
After the activation, you will be redirected to the plugin's Settings page.
All the information with your account detail you can find in BookIt Calendar > Account.