Summarize the following:
If you have an Amazon eBook that you want to add to your Kindle via USB, go to Amazon and log in if necessary, then do the following:  Select Account & Lists  Click Your Content and Devices  Click ... to the left of a book. Click Download & transfer via USB  Select your Kindle from the drop-down box. Click OK If your eBook isn't already in PDF, AZW, or MOBI format, you'll need to do the following before you can put it on your Kindle:  Go to https://www.epubconverter.com/epub-to-mobi-converter/ in your computer's web browser. Select the proper file-to-file converter on the left side of the page. Click Add File...  Select your eBook's file. Click Open  Click Start Upload  Click the MOBI file's link in the "Download Link" column to download it onto your computer. Click your eBook's MOBI file to select it, then press Ctrl+C (Windows) or ⌘ Command+C (Mac) to copy the file. Plug the charging end of your Kindle's charger cable into the Kindle, then plug the USB end of the cable into your computer. If you're using a Mac, you might not have traditional USB ports; if so, you'll need a USB 3.0 to USB-C adapter for your computer. The process of doing this will vary depending on whether you're using a Windows computer or a Mac:   Windows - Open Start {"smallUrl":"https:\/\/www.wikihow.com\/images\/0\/07\/Windowsstart.png","bigUrl":"\/images\/thumb\/0\/07\/Windowsstart.png\/30px-Windowsstart.png","smallWidth":460,"smallHeight":460,"bigWidth":"30","bigHeight":"30","licensing":"<div class=\"mw-parser-output\"><p>I edited this screenshot of a Windows icon.\n<\/p><p>License: <a rel=\"nofollow\" class=\"external text\" href=\"http:\/\/en.wikipedia.org\/wiki\/Public_domain\">Public Domain<\/a><br>\n<\/p><\/div>"}, click File Explorer {"smallUrl":"https:\/\/www.wikihow.com\/images\/0\/0d\/Windowsstartexplorer.png","bigUrl":"\/images\/thumb\/0\/0d\/Windowsstartexplorer.png\/30px-Windowsstartexplorer.png","smallWidth":460,"smallHeight":460,"bigWidth":"30","bigHeight":"30","licensing":"<div class=\"mw-parser-output\"><p>I edited this screenshot of a Windows icon.\n<\/p><p>License: <a rel=\"nofollow\" class=\"external text\" href=\"http:\/\/en.wikipedia.org\/wiki\/Public_domain\">Public Domain<\/a><br>\n<\/p><\/div>"}, click This PC, and double-click your Kindle's name.  Mac - Open Finder {"smallUrl":"https:\/\/www.wikihow.com\/images\/1\/1d\/Macfinder2.png","bigUrl":"\/images\/thumb\/1\/1d\/Macfinder2.png\/32px-Macfinder2.png","smallWidth":460,"smallHeight":431,"bigWidth":"32","bigHeight":"30","licensing":"<div class=\"mw-parser-output\"><p>I edited this screenshot of a macOS icon.\n<\/p><p>License: <a rel=\"nofollow\" class=\"external text\" href=\"http:\/\/en.wikipedia.org\/wiki\/Fair_use\">Fair Use<\/a><br>\n<\/p><\/div>"}, then click your Kindle's name on the left side of the window. Your Kindle may also appear on the desktop. Double-click the "Documents" or "Internal Documents" folder that's in your Kindle's storage folder to do so.  You may first need to unlock the Kindle and/or double-click the "Internal Storage" folder. If you're using a Kindle Fire, you'll open the "Books" folder instead. Click a blank space in the folder, then press Ctrl+V (Windows) or ⌘ Command+V (Mac) to do so. When the indicator light on your Kindle's cable stops blinking, you can proceed. This will ensure that your files are saved before you remove the Kindle from your computer:   Windows - Click {"smallUrl":"https:\/\/www.wikihow.com\/images\/7\/70\/Android7expandless.png","bigUrl":"\/images\/thumb\/7\/70\/Android7expandless.png\/30px-Android7expandless.png","smallWidth":460,"smallHeight":460,"bigWidth":"30","bigHeight":"30","licensing":"<div class=\"mw-parser-output\"><p>I edited this screenshot of an Android icon.\n<\/p><p>License: <a rel=\"nofollow\" class=\"external text\" href=\"http:\/\/en.wikipedia.org\/wiki\/Fair_use\">Fair Use<\/a><br>\n<\/p><\/div>"} in the lower-right corner of the screen, click the flash drive-shaped icon, and click Eject in the resulting menu.  Mac - Click the "Eject" {"smallUrl":"https:\/\/www.wikihow.com\/images\/3\/3f\/Maceject.png","bigUrl":"\/images\/thumb\/3\/3f\/Maceject.png\/30px-Maceject.png","smallWidth":460,"smallHeight":460,"bigWidth":"30","bigHeight":"30","licensing":"<div class=\"mw-parser-output\"><p>I edited this screenshot of a macOS icon.\n<\/p><p>License: <a rel=\"nofollow\" class=\"external text\" href=\"http:\/\/en.wikipedia.org\/wiki\/Fair_use\">Fair Use<\/a><br>\n<\/p><\/div>"} triangle to the right of the Kindle's name in Finder.

Summary:
Download an eBook from Amazon. Convert your eBook if necessary. Copy the MOBI file. Connect your Kindle to your computer. Open the Kindle. Open the "Documents" folder. Paste in the MOBI file. Wait for the MOBI file to finish transferring. Eject your Kindle.