Just keep an eye out. If you're jungling, ping the laning ally to hug their tower and let the enemy overextend. It will have a big impact and leave the opponent with little opportunities.
If it's a teamfight, again, just keep an eye out. Be aware of your enemies grouping, and ping when you find a good opportunity. You can ult behind the back line to sandwich them into your tanks and carries.
Fiddles is a hard champ to play, and finding the right time to ult is quite daunting. Making decisions with him is quite puzzling, but is very worth if you time it correctly.
Hide in bush > wait for enemies to be unaware of your presence > SURPRISE MUTHUFUKUS