If you're migrating from a blog that already has a custom domain name, you will probably want to get your new blog set up and looking spiffy before you point your custom domain at it. You can do this by using your temporary URL on Book Host.
In one of your order completion emails, you should have gotten the login details to your WordPress site along with your temporary URL address (it looks like this: http://yourblogname.bookhost.io/wp-admin/). That's where you can login to get started! (If you cannot find your temporary URL, please fill out a support ticket.) But, you'll notice that if you visit http://yourblogname.bookhost.io, it just redirects to your custom URL. You have to complete a few steps to get that working.
On your admin panel, go to Tools » Domain Mapping. You'll see your custom URL on that page. Click the trash icon to delete the domain mapping. Doing that will stop your site from redirecting to your custom URL. You can now view your site at the temporary URL http://yourblogname.bookhost.io. Use this time to prepare your blog for the migration.
Once you're ready to move your domain over, re-map your custom URL to your Book Host site. Go back to Tools » Domain Mapping and enter your URL in the box and save.