Well if ur having trouble landing Double or Nothing try to bring ur fingers as close to the yoyo as possible and you`ll land Double Or Nothing. Also u can go to yoyoexpert.com and go to learn and it is in the intermediate section on the learn button on the top. ;D
The more you do the trick the more the spacing comes naturally to you.
Yeah, i practiced it for like 2 or 3 days until i could land the yoyo consecutively on the string. Its a great trick to learn because it is a mount for a lot of tricks!