You could use DVD Flick or AVS2DVD. Both are freeware.
IMHO the first one is easier to use, you'll find a lot of guides, but it makes a single pass to encode the material. With the latter, you can get better quality, 2pass encoding and more parameters to adjust. If the difference in quality is noticeable, depends on many factors, such as the quality of the original mkv and the equipment where you'll watch the output DVD.
Give these a try and you'll find out what best suits your needs