These may have been lit by the mothers of the Bride and Groom at the beginning of the ceremony, or you may arrange simply to have them lit and burning as other candles are lit prior to the ceremony.

After the vows, or after being pronounced husband and wife, the two of you approach and each take up one of the lit candles. Together the you light the central, unlit candle.

There aren't necessarily words attached to lighting the unity candle. This is something you do, not something someone says. The unity candle may be something to do during a solo, particularly an instrumental solo.

Be aware that lighting the candle takes 30 seconds; the average solo takes 3-4 minutes. So don't start moving when the solo begins. Wait 60 seconds or so.

When you pick up the lit candle to use it to light the central candle together:
Be aware that the candle has been burning for 20-30 minutes and there will be a pool of hot wax around the burning wick. As you tip the candle, this hot wax will go somewhere; hopefully not on you. Pick up the candle and whatever stand it is in. It's easier to do that than to try to remove the candle from its stand.

If the 3 candles involved are in fact in 3 separate stands. Sometimes there's a unity candle holder that incorporates all 3 candles in one stand.

Also agree ahead of time about which of the two of you will really light the wick of the unity candle, and which of you will pretend to light it.

Pay attention to the choreography of lighting the candle. The two of you will have to approach the altar or table on which the 3 candles sit. The officiant may have to step out of the way for you to do this.

After you light the unity candle: