Buy and Arduino board, download the Arduino IDE, connect the board with a USB cable. Done.
Adding Board Selections.
Open the File/Preferences page, click on the icon to the right of the Additional Boards space, and paste the full web address of the JSON file(s) in the box. You get these from websites that document these other devices. After that, follow their insturctions from the website. This almost always seems to work.
Here are two that I have installed on both Win-7 and XP...
Other websites will send you to Github. Hit the Download button and extract the main folder to your desktop. From there, follow the insturctions on the Github page, or the original website, and simply move the main folder (loaded with lots of stuff) to the designated location - usually in a folder named 'hardware'. This method is less sure-fire. Sometimes you may need to put the folder in another location depending on your Windows version. Open File/Preferences and look at the 'sketchbook location'. Go to that location and look for a 'hardware' folder (create one if not there), and put the main folder into it.
In either case, next time you start the IDE the new boards will (hopefully) show up in the Tools/Boards menu.
(I have not gone into any great detail here, there are a lot of websites and YouTube videos out there that are very helpful)