Create a new page in RapidWeaver called “Under Construction”, design it, in the page inspector give it a filename underconstruction.html, leave the folder field empty, publish the site, then add the below to your .htaccess file at your web host to redirect all visitors except yourself to that new underconstruction.html page you just uploaded (you’ll need to replace the IP address in the below example with your real IP address, which you can get by doing a google search for what’s my IP).
If your ISP assigns dynamic IP addresses, you might need to update that IP on occasion while redoing the site, depends on how often they cycle your IP address.