You need to have Momentum available before the dice roll is made, if you want to buy additional dice.
Then you make the dice roll, determine the number of successes versus the Difficulty of the test, and any excess successes become (new) Momentum.
This Momentum can be used on Momentum spends for the currently resolved action or it could be stored in the Momentum pool.
Character A has to make a Difficulty 2 test.
The group Momentum pool has currently 2 Momentum.
Player of Character A decides to take 1 Momentum out of the group Momentum pool, to roll the test with 3d20 instead of the usual 2d20. The group Momentum pool now has 1 Momentum left.
The player rolls the dice.
The results are success, success, critical success for a total of 4 successes.
Compared to the Difficulty of 2, that results in 2 excess successes.
Those 2 successes become 2 Momentum.
If those 2 Momentum could be spent right away or stored in the group Momentum pool.
If they are stored, the group Momentum pool has now 3 Momentum left for the group to use later.
An assisting character does not generate any Momentum, but adds successes to the leading character for the action.
Those successes are added to give a total, provided the leading character has at least one success on their own roll, then the total number of successes is compared to the Difficulty of the test. Any exceeding successes become Momentum.
The test Difficulty is 2, so 2 successes are needed to succeed.
Charakter A is the lead, rolling 2d20 versus A’s Target Number, character B assists rolling only 1d20 versus B’s own Target Number.
A rolls 1 success.
B rolls a critical success on the single d20, adding 2 successes to the total.
The total number of successes is 3.
The Difficulty is 2.
That is 1 success exceeding the Difficulty, so this 1 surplus success becomes 1 Momentum.
This 1 Momentum could be spend right away or stored for later use in the Momentum pool up to the maximum of 6 Momentum.