1st of all have a large metal container.
Fill container with said flares
Stand back a big distance, put special device in metal container and expect a loud bang and lots of coloured smoke? Er....no Tom said it was more of a pop, fizz and a bit of black smoke. 20 minutes later - all gone!
Flares are dangerous, don't attempt this at home. The local collection point is the RNLI college (by appointment), it is illegal to dump flares in a public area or a Coastguard station.