Creating the bootable USB drive is actually step one. Old Toad provided a link to Apple's page, as did I. Here it is again:
https://support.apple.com/en-us/HT201372
Since you want to install Mojave, you first need an 8 GB USB flash drive. It seems you already have the Mojave installer on your drive. Just leave the installer where it currently is in the Applications folder.
You need this line on the linked page for Terminal:
sudo /Applications/Install\ macOS\ Big\ Sur.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume
First copy/paste this line into TextEdit. Change the MyVolume at the end to a , so it reads:
sudo /Applications/Install\ macOS\ Big\ Sur.app/Contents/Resources/createinstallmedia --volume /Volumes/a
Plug in your USB drive. Change the name of the drive to just the letter a so it matches the edit you made to the line above.
Now copy/paste the modified line that ends in a into Terminal and press enter. Enter your admin password. You'll then be asked if you really want to erase the volume named a . Type y and then enter. Everything after that is automatic. The USB drive will be erased and Mojave will be installed to it. When it's done, the USB drive's name will change to Install macOS Mojave.
You just have to wait a bit. A percentage progress will tick off. You'll know it's done when Terminal goes back to a new prompt that looks the same as the first one.
Close Terminal.
From here, we assume you have a current Time Machine backup and you've turned that drive off. Then we're back to the previous post. Though edited next to include more detail.
Boot to the USB drive with the older OS you want to install. Restart the Mac and immediately hold down the Option key. A screen will appear with drives you can startup to. Choose the newly made USB installer. This will take longer than starting up from the Mac's main drive. Be patient. Once booted to the USB drive, you'll see a minimal work screen.
Use Disk Utility (from the top menu bar) to erase the internal drive. A quick erase is all that's needed. Quit Disk Utility when it's done and then choose the option to install macOS.
As the OS install nears completion, have the drive with your Time Machine backup on. The OS will ask if you have data to restore. Say yes and choose the TM drive. It will restore your user account(s), any third party software you had installed, and all other personal data.