open up the doors or whatever you have to see where the pipes are under that sink. Under there you will see the drain pipe from the sink, then you will see what is called the P trap. This pipe makes a deep curve and then back up to the drain out of the house.
Remove the P trap fastening nuts with what is called channel lock pliers, this tool will expand out to loosen the nuts so that the P trap pipe will come out.
Take the pipe out, and remove your item. Caution, in that bottom of the trap will be all sorts of nasty gunk. the item will be imbeded in that so go through it in another sink with the stopper in. do not turn on the water in the sink missing the P trap until that pipe is back in place.
This should do it.
put the P trap back on and tighten it up and then check for leaks.