First open Media Encoder and bring in the file to convert
Drag it over to the Queue
On the first column (Format)
pull down the drop down and choose
DNxHR/DNxHD MXF OP1a
Choose DNX HQ or DNX SQ 
It is up to what quality you need and what aspect ratio the original file was in
Also choose what frame rate you need to match the original file or covert it
Then click on the Output Name to choose what to name and where to save the file
Choose to export audio as well here. Box is checked by default.
Leave Maximum Render Quality UNCHECKED it takes alot longer to export for marginal
benefit
Then click OK
to start export press the green arrow
