Read the DCS aircraft or vehicle manuals and review our Amplified Checklists provided with some of the Qualification Campaigns.
Watch the training videos and watch other videos that others have done.
Do some test flights or test missions of your own using the in game missions or Fast Mission Creator.
If practice missions are provided use them to get good at understanding what the Qualification missions will be like and what you need to do to pass.
Then when you feel ready, do the Qualification Campaigns to earn your Qualification.