The vote locking boost on provided liquidity has been enabled by the DAO contract.
To activate it, please vote-lock your CRV on https://dao.curve.fi/locker
You can then activate it on the gauge page on https://dao.curve.fi/minter/gauges or by performing any withdraw/deposit/mint actions.
If you lock more CRV, you will need to re-activate the boost to update it (or you can deposit/withdraw/mint to do so).
To calculate how many CRV you need to lock to achieve the maximum 2.5x boost, you can check out the calculator here https://dao.curve.fi/minter/calc
You can find a short FAQ on the vote locking boost at this address: https://resources.curve.fi/faq/vote-locking-boost