If you are planning on showing your longhaired cavy, you should train it from a baby to hold still, for several different reasons.

1) Learning to hold still on show board while being judged

Take a short box, tupperware, etc. that is just large enough for your cavy to sit on (when it's turned upside down). Set the cavy on it and make sure they don't jump off (they won't usually). You can give them a treat while they sit on top, but if you do and they get accustomed to it you may have problems with them holding still when they are being judged and can't have any treats.

When they have gotten good at holding still on the box/etc. you can move on to a show board. You can start out on a show board, but training first with the smaller box will let them learn that they can't move around.

2) Preparing your cavy for grooming

Once your baby has learned to hold still, you can get them used to the feel of a brush. Use the brush or comb and lightly groom them so they get used to the feel. You can also hold them, examine their eyes, nose, ears, teeth, and feet to get them used to being handled. Make sure you don't over groom your baby, too much brushing will cause unnecessary hair loss and coat breakage.