You can put it in an old sock and hang it in front of a return jet. Give the sock a squeeze every now and then to help it dissolve faster. Remove the sock before you add other chemicals through the skimmer.

Another method is to pour the CYA into the skimmer and let it dissolve in your filter. Just keep your filter running 24/7 for 4 or 5 days while it is dissolving and don't clean the filter during that time period or you'll just throw the CYA out.