Flame on!G 35

The game tells you this: "Spell out TORCH letters via Torch ramp shots to ignite the ball. If it's collected, Four Sign ramp shots must be completed with flaming balls to complete the sign and start this multiball mode."

Here's how you do it.
To activate the TORCH letters, you must get the ball all the way up the small ramp that is slightly hidden under the right hand side ramp. If the ball does not go all the way up the ramps, or you get credit for a "trick shot" which comes down on the left side, you will not get credit for the Torch ramp.

You will know you're getting close, because when you get TORC, Johnny will shout "Flame on!" (not to be confused with the "Flame on! Skillshot!")

Once all five shots have been made, Johnny will fly over to the sign ramp (the ramp leading to the 4 symbol with Doom on top). You must hit the ball up this ramp four times, igniting the lines of the 4 symbol. After some time, the ball will lose its flame. When this happens, hit it up the Torch ramp again.

I've found what usually works for me to get the ball into position for the Torch ramp is this:
From the left paddle, hit the ball through the INVISIBLE ramp/orbit all the way around.
From the right paddle, hit the ball into the hole just left of the sign ramp (where you'll start missions if you've gone into the Baxter Building) then hit it with the top right paddle when it pops out.
Try and get it to follow the line of flame drawn/painted on the board.

This is how I unlocked it on the 360 version.