Any musician with fairly advanced knowledge of chords knows how to substitute a dominant seven chord like turn
C G F C to C D7 G F C But how would one go about using major seven chords to substitute? Other that the same way you substitute a dominant chord. :mellow:

I tend to use them as embellishments, or a tool in order to get the lines to work smoothly... with major sevens, most don't use them in a functional sense outside of pop and jazz... however, I do love a good major seven. I use them too much... however, I am no jazz musician. In the style within which I write, it is uncommon to choose to use a major seven to replace a completely separate chord, keeping the harmonic function. I'll try to ask a jazzer, so I can reply better...

They can be used instead of dominant sevens. Just keep the note that would be the 7 of the dom. 7 and add the major seven that would have that note as the 7, or the major nine in the same way.
However, I still stick with the notion that they're not really used as functional.
Sorry for the lack of clarity :P