This next seven chord is quite useful as it can be moved up and down the neck like the F7 chord.
The way I like to look at it is that the C7 position sits on top of the chord that is being played in the B finger position.
So, if you are playing a C chord in the B finger position on the third fret, the C7 sits right on top of it. I mention this because it is common in songs from the blues to the Beatles to go from a major chord to a major seven chord at the end of a chord progression. Practice playing the C7 chord then practice playing the C major chord in the B finger position on the third fret and transitioning to the C7 chord.
Once you are used to this, move it up one whole step to play D major and D7 using the moveable B finger position and the C7 position.
Don’t stop there! Move it on up another whole step, rinse and repeat!