I own two accounts, can I use the soulstone to move skills between my own characters on different accounts?
No, the soulstone is account specific and you cannot move skills between two accounts even if you own both of them.
I read that you can use soulstones to make your character a blacksmith one day and a warrior the next (ie. switch back and forth between two professions), how do I flip-flop between two skills with a soulstone?
This is a somewhat misleading statement that was originally put out with soulstones. A soulstone only holds one skill at a time, so to flip-flop between two skills there are two possible methods:
method one - The Two Soulstone Method:
Say you want your character to be a smith one day and a fisherman the next, but do not have room for both skills at the same time (if you had room you wouldn't need a soulstone!). You would store blacksmith on one stone and fishing on the other, to go from fisherman to smith you would store fishing on stone A and add smithing from stone B. To switch back, put smithing back on stone B add fishing from stone A.
method two - The Two Character Method:
This assumes you have only one soulstone and you have another character with room to hold BOTH skills at once. Remove fishing from character A, store it on the soulstone. Log in character B (the one with room to hold skills) and move fishing to character B. Then 24 hours later move smithing from character B to the soulstone, log in character A, take the smithing skill from the soulstone to character A.
To switch back, remove smithing from A to the soulstone. Move smithing from soulstone to B. Wait 24 hours. Move fishing from B to the soulstone, then fishing from soulstone to A.
I only have one soulstone and none of my characters have room to store skills, what the heck good is this thing?
The main use for soulstones, if you haven't got any room for skill storage on other characters and only have one soulstone is to give you a backup for trying new skills.
Say that I have a mage with the wrestling skill, and all her skill points used. I want to try swords on her instead of wrestling, but just in case I don't like it, I don't want to have to work wrestling all the way back up again. I can store wrestling and pick up swords. If I like swords I can flush the stone to use it for something else. If I don't like swords after all I can set it to drop, use the soulstone, and be right back to where I was before without having to redo wrestling.