To become thoroughly familiar with the various 7th chord types in every key, you will need to write them out starting on all twelve notes of the chromatic scale (using both sharps and flats).
Work on one type of 7th chord at a time until you know them all. It is also important to practice them on your instrument as arpeggios around the key cycle and chromatically up and down.
A major seventh chord is created by adding a major 3rd on top of a major triad. As with other chord types, learn which notes make up the chord and then practice it as an arpeggio across the range of your instrument as shown with a Cmaj7 chord in the following example.
Once you start to feel comfortable with the major 7th arpeggio, try improvising with it.
As with everything you learn, it is essential to be comfortable with major 7ths in every key.