Indeed, it's easy to convert DVD to MKV and almost all ripping software share the common steps (Load DVD content > Set Profile > Get Start). The following is to show the steps with WonderFox DVD Ripper in detail:
Step 1:
Download and run the software
Step 2:
Load DVD content and set profile
2-1, Load video and select the video you want
2-2, Set profile to MKV under "Common Video"
2-3, Set destination folder
Step 3:
Start the process
3-1, Click "Start" to run the conversion process
3-2, After process done, click "Find Target" to get the output

How to Convert DVD to MKV, Rip DVD Video to MKV