From the Sink Method blurb:
Sink Method: Disassemble your pen near your sink or cleaning station and empty the ink. If the pen is full, you can try emptying the ink back into the original bottle or another small vial to save it for future use. If you’re not worried about that, you can simply empty it into the sink. Next, hold the nib section under running water until the water runs mostly clear. This is the fastest way to flush your pen, and for pens that are properly maintained and haven’t accumulated buildup or debris, this step may be enough.
> If the pen is full, you can try emptying the ink back into the original bottle...
Please note that doing this may introduce contaminants into your beloved ink bottle, which in turn could promote fungal growth and other nasties. Best not to put ink back into a bottle. As an aside, I use a blunt-tipped syringe to pull ink from the bottle for filling - none of my pens ever come into direct contact with a full ink bottle.
Tip: For the sink method, it is recommended that a stopper be placed over the drain hole while cleaning so that disassembled parts don't find their way down the drain. Feeds, nibs, rings, and o-rings can easily find their way to a dark place otherwise.