I'm amazed no-one replied.. I guess chum is more of an offshore thing. I don't claim to be a chum expert, most of the time I buy my chum, it's nearly cost effective to just pay the $8 a gallon or $20 for 5 gallons.
I never fish flounder either, so just use what you can. I'm basicly just replying because no-one else has.
A quick & dirty chum method is simply a can of dog food in panty-hose or laundry bag. This is good for blues, mackeral, and probably everything else. I actually even use dry cat food (seafood flavored) mixed in w/ vegatable oil..
To make your own, get fish oil if you can (if not substitute w/ vegatable), some type of fairly fresh bait (in my area herring is $12 for a 20 lb flat) and an old school meat grinder. Grind up the bait as small as reasonable & possible, add the oil & some water, mix & freeze in no larger than 1 gallon tubs.
The reason no larger than 1 gallon is that in normal freezers it will begin to rot in the middle before it freezes. Rotten chum is no good.
Throw this in a laundry (or chum) bag or chum bucket (minnow traps work too) over the side. As it dethaws it will dispense. For flounder you will want to sink it to a bit above the bottom (if drifting you don't want it to get caught). For any species that will come above bottom, float the chum next to or nearby the boat. Put the bait in the chum somewhere.