Code:
A - A#/Bb - B - C - C#/Db - D - D#/Eb - E - F - F#/Gb - G - G#/Ab - A
fret 1 2 3 4 5 6 7 8 9 10 11 12
this is how i picture it in my head. those 12 notes are basically your twelve initial frets on a stringed instrument (like on guitar). so in this case, you have an A string so when you get to the 12th fret you are back to an A again.
but you can use this for transposing any chord without having to learn what key you are playing in. its more work, but will help you understand how to transpose.
so you started with F, C, and Em.
take your F. its number 8 on the line there. you want to transpose everything down 3 half steps. 8-3=5. so now all the F's in your song you are going to change to D's. if you have to, print it out, cross out all the F's and write in D's in place.
now C. its #3. 3-3=0. the 0 position, of course is A. so now all your C chords will be A chords.
now here is where it might get confusing. you have an Em. dont worry about the minor just yet. just take the E for now. #7. 7-3=4. now we have C#. now time to remember the minor. so all your Em's are now going to be played C#m.
if this is a chord pattern that takes up your whole song, then youre done. just remember instead of F, C, Em... now you are playing it D, A, C#m