First, you’re going to want to snag yourself a copy of RebootWP.
Download from WP.org or grab the .ZIP from Github.
Log in to your WordPress Admin section at https://your-website.com/wp-admin/
Once logged in you will need to navigate to Appearance → Themes → Add New. from the left menu.
From there click Upload from the top sub-menu.
Click on the Browse button and browse to where you saved your theme and select it.
Now click Install Now.
Once installed it will return and ask you if you would like to activate the new theme, click Activate.
Once activated, you’re ready to go.
Head to Appearance → Editor to begin designing your site.