how to do a clean installation of Mojave?

A is the right one to use: Factory reset of your Mac - Apple Support.

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How To Do A Factory Reset

Selection A should be used on computers that came with Lion or later when factory new. These models had no disks included when new. Selection B is for Macs that came originally with Snow Leopard or earlier. These models shipped with Software Restore disks when new.

A. Factory reset of your Mac - Apple Support

B. Factory Reset Your Pre-Lion Mac

Follow these instructions until you get to Step 5 of Factory reset of your Mac - Apple Support. At Step 5 you will need a Snow Leopard DVD or the installer disc that came with the computer.

1. Boot the computer using the Snow Leopard Installer Disc or the Disc 1 that came with your computer. Insert the disc into the optical drive and restart the computer. After the chime press and hold down the "C" key. Release the key when you see a small spinning gear appear below the dark gray Apple logo.
2. After the installer loads select your language and click on the Continue button. When the menu bar appears select Disk Utility from the Utilities' menu. After Disk Utility loads select the hard drive entry from the left side list (out-dented entry - mfgr.'s ID and drive size.) Click on the Partition tab in the Disk Utility main window. Set the number of partitions to one (1) from the Partitions drop down menu, click on Options button and select GUID, click on OK, then set the format type to MacOS Extended (Journaled), then click on the Apply button.
3. When the formatting has finished quit Disk Utility. Proceed with the OS X installation and follow the directions included with the installer.
4. If you are planning to sell or give your computer away, then do the following: After you reformat your hard drive and reinstall OS X, the computer restarts to a Welcome screen and asks you to choose a country or region. If you want to leave the Mac in an out-of-box state, don't continue with the setup of the system. Instead, press Command-Q to shut down the Mac. When the new owner turns on the Mac, the Setup Assistant will guide them through the setup process.

I’ll state my agreement with Mr. Eckert. “Clean installs” are a remnant of the old days and a carryover from the Windows world where periodic erase and install routines were common practice. I currently have a Late 2013 iMac that shipped with macOS 10.8 Mountain Lion and have continually upgraded to 10.9, 10.10, 10.11, 10.12, 10.13, and now 10.14.1 Mojave without performing any clean installs. No problems or issues of any importance.

Alan,

No offense but your remarks aren't of much help since you don't know why the OP wants to do a factory reset. Doing an Erase and Install is not a waste of time when done for the right reason.

Kappy

I asked why he thought he needed a so called "clean" install before posting my reply.

The reason I call it a so called "clean" install is because in my experience hardly ever are they truly clean installations.

Yes, I do know that for the right reason, there are time when it is necessary to do such a thing but general I have found they are being resorted to far more then it really needed.

What do you expect to gain from a so called "clean" install of Mojave?

It has been my experience that doing them is a complete and total waste of time except for a newly purchased second hand Mac.

In a future world I might agree with you, but in the world of today, I say Nonsense. You cannot generalize from a single observation and with no standardization.