If you send me an email to firstname.lastname@example.org
(I can reply quickly with an attachment) I'll send you some material I use for teaching the basics, it includes transposition info using a capo. It's relatively simple and you could do it in your head, but if you have a reference system it's useful when you're brain is full or you are playing in a band/group and the key changes. Not all Keys can be transposed easily as there is a limit to how far up the neck a capo can go before it sounds tonk(Some say no further than the 7th fret). In the A to G example, simply put your capo on the 2nd fret and play the sequence using the chords in G instead of A. If you want to send me details of the song you're trying to transpose I'll try and help with that too.