Summarize:

Go to https://www.ubuntu.com/download/server in your computer's web browser. It's a green button on the right side of the page. Your Ubuntu Server disk image (ISO) will begin downloading onto your computer. Go to https://rufus.akeo.ie/ in your computer's web browser. Scroll down to the "Download" heading, then click Rufus 3.1 below the heading to prompt the launch file to begin downloading. Insert a flash drive that has at least 4 GB of storage capacity on it into one of your computer's rectangular USB ports. Keep in mind that the flash drive will be completely erased during the installation drive creation process. You may want to copy any existing files on the flash drive onto your computer before proceeding. Double-click the downloaded Rufus file, then click Yes when prompted. It's on the right side of the Rufus window. This will open a File Explorer window. Go to the location of the Ubuntu Server file, then double-click it to open it in Rufus. It's at the bottom of the Rufus window. Your ISO file will begin burning onto your flash drive. You may have to click through a few prompts (e.g., click OK to begin the process) before your flash drive will begin burning. This may take several minutes. Once your flash drive finishes burning, you can proceed to the next part. Don't remove your flash drive once the burning completes. You'll need to keep your flash drive inserted throughout the installation process.
Open the Ubuntu Server download page. Click Download. Open the Rufus website. Download Rufus. Plug a flash drive into your computer. Run Rufus. Click SELECT. Open the downloaded ISO file. Click START. Wait for the burning to finish.