So you have a song and now you want to add a vocalist. As usual, the singer cannot sing in that key.
Now you have to transpose every chord. Of course, if your environment is completely electronic, it's simple, just press a few buttons or knobs to adjust it.
Here is an easy tip, you can quickly create a transposition map on a piece of paper.
For example, let's say the original song was written in the key of F, and you want to move to Ab.
On the first line write every note on the key of F (major). Even if the song is mostly in minor chords, just use the major sale for this purpose.
F G A Bb C D E
and now on the second line write down all the major notes in the new key. It is also a good time to check against the number of flats or sharps on the musical staff if you have one, for example, in this case we are talking about Ab key which is has 4 flats.
F G A Bb C D E
Ab Bb C Db Eb F G
Now, it is easy to transpose, if your first chord is Fm7 then the new chord would be Abm7.
If you a B7 chord in the original, that's half step above Bb so you can simply add half step above Db which comes out to D7. Note that for Jazz chords like F#m7b5, it is just a matter of transposing the letters. In this case F maps to Ab so F# is a step above F so half above Ab is A, thus it would be Am7b5 when transposed.
You can also use the above map to transpose the melody line easily.
A side note: These b5, #5, b9 stuff is called dominant chord (the one with 7 in it) alterations. If you don't know how to play them just play the 7th chord indicated and omit the alterations and hope either other band members would play these notes, or it's often in the melody of a song in which case you would want to avoid playing as it will crash with the vocalist.
Have fun!
