In Audiosurf the achievement Candy Stripe has the requirement "Make a candy stripe". What configurations count for this achievement and what mode should I use to get it?
A candy stripe is alternating red and white blocks in one lane. You must play on at least medium difficulty (or Easy, and set the "color scheme" to "black" in the options) for white blocks to spawn. White blocks are relatively rare.
One way to do this is with a Pusher, on medium or hard, as shown here:
The Pusher is capable of moving blocks from one lane to another, so you can ignore all the blocks except for the red and white ones, and just drop them into the same lane. Since you're not going for score, you can stay on the shoulder for most of the level.
If you're making your stack in the middle lane, and you want to move a block onto the stack from the left lane, line up your Pusher with the block and then hold X (or the Right Mouse Button if you're using the mouse) as you pass it. For blocks in the right lane, line up and press Z, or Left Mouse Button.
You can also use Pointman Pro, if that's more your speed:
Holding left mouse button will hold a block you're about to go over, and pushing right mouse will drop it.
I'd rate this as "easy" as far as the holiday achievements go. Total time invested is probably going to be less than 15 minutes even if you're new to Audiosurf. If you're having issues, you might try playing a slower song. Slower songs tend to spawn fewer blocks per second, but be wary of picking a song that's too short or too slow, lest you run out of blocks before the song is over.