you can put the bleach (full strength) directly into the containers and swish it around - or use an old rag to make sure all parts of the container are wet down with the bleach.
rinse with hot water if you can...cold is okay, tho.
for the cat pan, do the bleach treatment and then get some of the enzyme cleaner that removes urine/feces.
make sure the bleach has been rinsed away - i always rinse at least 2 times - because the enzyme stuff will be nullified by any residue that's left from other cleaners/soaps.
follow the package directions. usually you need to shake the container well and then apply the stuff. again with a rag to get the liquid on every surface and then let it sit so the enzymes can do their thing with getting rid of the feces/urine. usually the wait time is about 30 minutes. then rinse with clear water and let dry.
yes, there's still stuff there, even after treating with other cleaners!! and you don't want any of that stuff in the container when you put your plants
in it...cats will still be able to smell it, even if you can't!!
also, if you want to use that container for growing
foodstuffs, you definitely need to remove ALL traces of urine and/or feces before doing so.