Hi,
Depends how you are getting your 12v now.
If its a 12v battery,
Then you need another 12v battery to make 24v.
If its a transformer from the mains with a split
winding feeding two diodes for a 12v supply,
then you could use a diode bridge and ignore the
centre tap to get 24v.
If its a switching supply with no transformer
then get someone else to do it.
If its a transformer from the mains feeding a
diode bridge for a 12v supply, then
you need a re-wind or a different transformer.
If its none of the above, then give details.
John