| Never done it on a Duke but, I guess it's pretty much like any other bike.
Pump the lever enough to push the pistons nearly all the way out.
Pull the pistons out, note which caliper and position they came from, drain all fluid from the calipers and the master cylinder.
At this stage I prefer to disconnect the calipers from their lines and work on a bench.
Remove the seal and discard.
Clean the calipers inside and out thoroughly using just a rag or if you prefer brake cleaner.
Clean the pistons thoroughly, remove any roughness by gently rubbing them with scotchbrite or similar. Check them for any signs of scoring or other damage.
Make sure the calipers are completely dry and free of any dirt or residue cleaning fluid.
Wet the new seals with clean new fluid, put them back it. be careful not to get the seals twisted in their grooves.
Wet the pistons with clean new fluid and push them back into the calipers.
They should slide in quite freely - if they don't then you have the seals either twisted or in the wrong way round and possibly the piston not straight.
Don't push the pistons in all the way though as the new seal might pull them back in again when you release the pressure when bleeding the brakes - this will make it difficult to bleed them effectively.
Use new copper washers when refitting the brake lines.
I won't bother describing how to bleed brakes - there's plenty of good info out there.  | 
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