Once the Mojave installer has finished downloading, select the Mojave icon in the macOS Mojave Patcher window.

Select your Mojave installer and click open

Now, select your volume by clicking the dropdown menu under the picture of a hard drive. You want to select the name of your USB drive.

My USB drive is not Macintosh HD. Do not select your Mac's hard drive/solid state drive/solid state hybrid drive. Instead, select your USB drive's name.

Click start operation. This may take a while to complete based on your drive speed.