You can do a search for "canna seed" in the search box and likely get good information.
You do need to know that the seed capsule needs to dry (turns brown) so the seed inside will fully mature. There are a number of hybrid cultivars that are sterile and "will not" produce viable seed. This is typically indicated by the size of the contents of the seed capsule. Mature canna seed is about the size of a green pea but will be brown to jet black. If the contents look like small mustard seed, fertilization was incomplete or unsuccessful.
Please remember that hybrid cannas do not breed true and the offspring will not be a clone of the parent. Do not call the hybrid canna seedlings by the parentÂs cultivar name.

The below link is very useful.
http://growingcannasfromseed.blogspot.com/2009/03/growing-cannas-from-seed-can-be.html

Kent