As a general rule you can use the minor 7 chords to replace normal minor chords.
The major 7's can replace normal major chords IF they sound good. Sometimes they sound too 'jazzy' and it's a matter of using your ear to see if it sounds good.
They can be used as what's known as 'substitutions', to replace any normal major chords. Once again it depends on if it sounds good. Sometimes the major 7 chord is too 'strong' sounding and will clash with the vocal.
But be careful if you are doing a cover of a well known tune and replacing chords with extended chords or you may get funny looks from the rest of the band  .
Dominant 7th chords can be used to great effect in most Blues style progressions. For example instead of playing a standard 'A, D and E' 12 bar progression, you could use A7, D7 and E7 to add flavour to the progression and make it sound much more 'authentic' and Bluesy.
The Beatles used them a lot and it's worth studying the chord progressions of their famous songs to see where they fit in well.
A study of classic tune progressions is a good place to see how these type of chords are used. In fact studying well known songs and the chords used
is a great way to expand on chord use and knowledge.
Search on the Net for the chord progressions of songs you like and see how they are used. A few example Beatle songs that use these type of chords to great effect would be:
'Something'
'Yesterday'
'When I'm 64'
When it comes to writing original stuff you can use these extension chords to add flavour to any progression. Experimentation is the key here.
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