Provided you don't actually still have the installer somewhere on your mac, you should be able to redownload it from the purchases tab in the mac app store. You have to sign in though to see the purchases first.
Of course if you still have the installer somewhere the redownload is not necessary. The first thing I do after downloading a full installer is to back it up somewhere so that I never (or rarely ever) have to redownload it.
You may then proceed as per this thread to make a bootable usb:
https://forums.macrumors.com/threads/make-bootable-usb-drive.1890564/
Then boot up from it using the option key and use disk utility to wipe the whole drive. Ensure you have a backup before doing this. Then use the usb stick to do the clean install.