It is a pretty simple process to convert MKV to DIVX. The conversion can be performed by a number of programs readily available on the Internet. Once a program is chosen, the steps to convert are pretty straightforward. Before launching the conversion program, however, it is recommended to install the Combined Community Codec Pack (CCCP), which can be found for free on the Internet. This pack contains the the DivX compression filters necessary to convert MKV to DIVX as well as other filters that you may use in the future.
Once the CCCP is installed, the first step to convert MKV to DIVX is to pick your converter, install it, and open it up. After you start the converter, you should see a toolbar or a menu bar that has several options featured on it. Somewhere among this myriad of choices is a button that says "Add files" or something similar. If you don't see such a button, check the menu bar to see if there's a "File" tab. If there is, click it and there should be an "Open File" option in the drop-down list. The program will then ask you which file you would like to open.
At this point, you should either select a specific MKV file to convert, or use the "Files of Type" drop-down menu to select the .mkv file extension and browse through your MKV files. Once you have chosen the file you would like to convert, click "Open", "Next", or whatever button indicates that you have made your choice. If you haven't installed CCCP, you may not be able to progress any further.
Your converter will then ask you to select an output format from the list of available file types. Different converters have different output file types available, but generally, all of them should have a DIVX. If this isn't an option, you may need to find and download another converting program that specifically indicates it will convert MKV to DIVX.
At this point in the conversion process, a few converters have the option to change certain video and audio settings for the finished product before you confirm the conversion. If this turns out to be the case, it's recommended that you keep it as simple as possible and just go with the defaults. Unless you know exactly what you're doing, you'll probably end up doing more harm than good by messing with the settings. Lastly, choose your destination directory, click "Convert," and wait for the program to do the conversion.