When Rek'Sai Breacher accelerates a token, how do you pay the accelerate cost?

When you use Rek'Sai, Breacher to grant Accelerate to a token, you pay the cost as part of the process of playing that token.

According to RiftJudge FAQ #9271, FAQ #8671, and FAQ #9360, here is how the payment works:

1. Triggering the Play: When an ability instructs you to "play" a token, that token is considered to be played from a zone other than your hand. Because of Rek'Sai, Breacher, that token gains the Accelerate keyword.
2. Optional Additional Cost: Accelerate is an optional additional cost. As you are playing the token, you must decide whether or not to pay the cost to have it enter the board ready.
3. The Cost: The cost for Accelerate is [1] Energy and 1 Power.
   • Per Rule 805.1.a.2, if the unit has no domain or more than one domain, the Power portion can be paid with [A] (a Power of any domain).
4. Payment Timing: You must pay this cost during the "Pay Costs" step of playing the card (Rule 357). You cannot pay the Accelerate cost later once the token is already on the board.

Sequence:

• An ability (such as a Deathknell or a "When you play me" effect) instructs you to play a token.
• As you are playing the token, Rek'Sai, Breacher grants it Accelerate.
• During the "Pay Costs" step, you choose to pay [1] Energy and 1 Power for the token.
• The token enters the battlefield ready. If you choose not to pay, it enters the battlefield exhausted (the default state).

Note: If you are playing multiple tokens at once (e.g., from an effect that plays two tokens), you pay the Accelerate cost separately for each token and may choose to pay for one, both, or neither.