You'll often see alt chords in a minor progression, so let's say we're in the key of F minor. The V chord is C7, the notes are C E G Bb. The C7 resolves nicely to the F minor, but sometimes composers want a stronger resolution. You can do that by adding notes that add tension to the chord. But how do you know what notes to add?

Let's do this by process of elimination. You won't use D, F, or A. These are the 9th, 11th and 13. The 9 and 13 sweeten the sound of the chord, which is the opposite of what we want, and the 11th goes badly with the E. It's an avoid note.

You also won't use B. That is the major 7th and we already have a dominant 7th. Also, we will exclude the fifth, G. Many of the overtones found in the G are already in the root, C, so it adds little to the chord.

This leaves us with C, E, and the five black notes on the keyboard, including the dominant 7th, Bb. Db and Eb are the b9 and #9. Gb and Ab are the #11 and b13. You can call these b5 and #5 and in fact you'll see it both ways. All of these notes put together make up the altered scale.

Adding any of these notes will increase the tension of the V7 and lead to a stronger resolution to the I. Try it! In your left hand, play C E Bb (root, third and seventh). while playing an Ab major chord in your right hand (or Gb or Dbm). Invert as necessary. Now resolve the chord to an F minor. Nice!

Okay, now let's talk about ii V I using A7 as you described. In a minor key, that would be Em7b5, A7alt, Dm7. (You can also use Em7b9, and for the Dm you can add the 6 or the 9 and even the maj7.) The goal is to have smooth voice leading, so use notes that progress nicely from one to the next.