Successes

When you succeed at a roll, you "spend" the successes you gained from the roll in order to determine exactly how you succeeded. The first success is always used to accomplish the primary task at its most basic level--to hit your opponent, find a clue, or leap over the fence. Additional successes can be used to improve upon the success either by adding descriptive flourishes, increasing the effectiveness of the main action, performing additional actions as part of the main task, or performing the task in a way that has beneficial or interesting side-effects. While most subsystems for specific types of action (such as combat or spellcasting) have their own rules for exactly how successes may be spent, there are some general guidelines that work for most types of rolls.

Increase Effectiveness

If the effects of the action are measured in either game mechanics units (like Hit Points or ticks on a tracker) or game world units (like feet or minutes), you can spend successes to modify these units in your favor. Examples include increasing the damage caused by an attack, decreasing the casting time for a spell, or moving a persuasion tracker one tick closer to where you want it. The successes to units ratio may not always be 1:1, and in some cases spending multiple success may have diminishing returns, with the cost per unit increasing or the units per success decreasing with each additional improvement. For example, the GM may allow you to add a foot to your vertical jump by spending a success, but just 6" for the second and 3" for each success after that. Assuming the initial success gives you an average jump (17"), spending 1 success to improve effectiveness would put you on a par with the average NBA player (29"), but you'd still have to spend 4 more successes to jump higher than Michael Jordan in his prime (44" to Jordan's 43"). You'd need 7 additional successes (for a total of 13, counting the initial success) to match the world record of 65".

Multi-Task

You can trade some of your successes in for bonus actions, provided those actions are things that the character could reasonably accomplish alongside the main action. For example, a character involved in a foot chase could use successes to reload a weapon, knock over a trash can, or jump a fence. Actions that do not require a roll typically cost 1 success each, though the GM can set a higher price for actions that are especially difficult to perform while also doing the main action--for example, the GM may decide that sending a text while involved in a car chase costs 2 successes. For actions that require a roll, the player must spend the number of successes the want for the secondary action plus 1 success for each roll-requiring action already taken; in other words, a second roll action costs 1 + successes for action, the third costs 2 + successes for action, etc.

Add Embellishments or Flourishes

You can spend a success to add a cool descriptive details. In some cases (like when you add unnecessary acrobatics to a movement roll), this is similar to multi-tasking, but you can also use flourishes to expand on how or how well you succeeded (describing how your performance in the Battle of the Bands had women feinting in the audience) or how cool you looked doing it (detailing how the light played across your mighty thews as cleaved the snake creature in twain).

Give Yourself or Allies Bonuses

If you can justify how your success gives yourself or an ally an advantage, you can trade success for bonuses to appropriate rolls. Bonuses must be more narrowly-defined than a Trope or Trademark. For instance, you can't give your ally a bonus to all combat rolls, but you could give them a bonus to all combat rolls against a specific opponent or all combat rolls made with a specific weapon. 
If the bonus only applies to rolls made in the next round, it costs a number of successes equal to the bonus. Multi-round bonuses are always treated as fleeting bonuses with a cost equal to twice the starting bonus.Non-fleeting multi-round bonuses and bonuses that remain in place for the rest of the scene (or longer) are possible, but these cannot be player-generated. They only occur when specific game rules (the effects of a spell, for instance) come into play.

Give an Opponent Penalties

The cost for giving an opponent penalties in is the same as for giving yourself or an ally bonuses. In some cases, the opponent can reduce or negate the penalty by using their action to correct whatever is causing the issue. For example, if an NPC has a penalty to movement rolls because you pulled their pants down, they can use their action to pull their pants back up and negate the penalty. If the GM rules that attempting to negate the penalty requires a roll, the NPC must roll a number of successes equal to the penalty dice in order to negate the penalty. Whether or not a partial negation (reducing the penalty by the number of successes) is possible is up to the GM based on what's causing the penalty.

Affect a Tracker

If you can assist or impede something being measured on a tracker, you can move the tracker up or down, usually by 1 tick per success. Examples include using successes from combat rolls to force an opponent into a certain position (so they're at a disadvantage or so your team can spring a trap) or using successes from a movement roll to keep a countdown tracker from advancing because you bought time by making it from point A to point B faster than expected.

Trade for Instant Karma

Finally, you can trade 2 successes for 1 point of Instant Karma.

Maintained Bonuses and Penalties

Some bonuses and penalties are based on player actions that can be continued over the course of several rounds. For example, and opponent may continue to suffer penalties to vision-based rolls for as long as a player keeps their flashlight trained on their face, and NPC may suffer movement penalties for as long as an opponent keeps them in a half-nelson, or an ally may continue to benefit from increased strength for as long as the wizard concentrates on their strength spell. These bonuses and penalties remain in effect for as long as the relevant player continues to take the appropriate action. Maintained bonuses cost the same as a 1-round modifier if they end as soon as the maintaining action is abandoned. If they linger a bit, treat them as fleeting actions when determining cost.