You can get the balls and patches you need from Track of the Wolf.
http://www.trackofthewolf.com/Categories/TableList.aspx?catID=2&subID=25&styleID=58&PageSize=25&Page=3Go ahead and use the Cannon grade for now. Nothing wrong with that.
If you refer to GOEX's load chart
http://www.goexpowder.com/load-chart.html you will find they suggest FFg in a range of 70-80 powder measure setting for .69. Matthew Switlik suggests in his book that you reduce loads ridiculously to start. I suggest you start at 40 grains and work up.
In the small cannon, the ball can go in tight. One inch or larger should have windage. The .690 ball will work but may lead. You might need a lubricated over powder wad with bare ball. For a .678 ball you will need a .010 patch to start.
Procedures for shooting my little cannon is different than shooting my larger cannons and is different than shooting big cannons. For this discussion your cannon is a little cannon.
Tools I use for shooting my little cannon-.50 cal
I have two ramrods. One for seating ball. A second rod has a old 410 bore brush on the end that I use to swab the bore. You might try a 20 guage brush. An old wore out brush works better for this than a new brush. I also have a small 17 cal bore brush on a home made handle for cleaning the vent. A vent pick made from a round wooden tooth pick or brass rod is also useful.
I will go through my loading and shooting process for you. Before I go to the range, I premeasure my powder and put them in small plastic tubes.
The procedure.
1. Clear the vent with the vent brush.
2 Wrap a clean dry patch around the 410 brush with the patch over lapping the end of the brush. Run the brush down the bore to wipe and dry the bore, and verify the bore is clear. There should be a mark on the rod showing how deep the rod goes in on an empty barrel.
3. Insert the vent pick in the vent. If you use a fuse or quill, you can use that instead. This will keep the powder from going out the vent hole when the barrel is charged.
4. Tip the barrel up and pour the premeasured charge down the bore. Be very careful to control the muzzle when you do this and not let the muzzle point at you or anyone else.
5. Place a patch centered over the bore and place a ball on the patch, and seat the ball on the charge. The seating rod should have a mark on it showing when the ball is fully seated.
6. Place the gun back on line and if not using fuse or quill, prime the vent.
7. Aim the gun
8. Fire
9 clear the vent with the vent brush
10. Wrap a wet cleaning patch around the bore brush and swab the bore.
11. Wrap a dry patch around the bore brush and dry the bore.
12 Load
You still have time to get in on the postal shoot.
Postal Shoot . There is a link in the postal shoot post to aiming a cannon.
If you have any questions do ask them here. We will be glad to help.