Summarize:

Flip your piece of coroplast over so your scores are face down. Slowly bend the walls up along the scores until the walls form a 90-degree angle to the base. Make sure the corners sit flush with one another. If the corners of your walls don't line up, either trim them if they're too long or tape additional coroplast if you need to fill the gap. Cut 6in (15cm) strips of duct tape with a pair of scissors. Use 1 strip in each corner of your coroplast base to hold the walls together. Place the tape on the outside of the coroplast so your guinea pigs can't chew on it. Have a partner hold the walls up while you tape the corners so they're less likely to fall apart. Put the coroplast on the bottom of the cage so they cover the grid. Push the coroplast base down into the cafe until it's flush with the grid pieces underneath it. Cover the bottom of your new cage with aspen, kiln-dried pine, or wood pellet bedding so your guinea pigs can be comfortable in their new home. Since guinea pigs need places to sleep away from light, provide igloo covers or plastic tunnels so they can hide. Line sleeping or hiding areas with fleece so your guinea pigs are even more comfortable. Provide more than 1 hiding area if you're housing multiple guinea pigs. Avoid using aspen shavings since the aromatic oils can cause severe respiratory problems. Transfer your guinea pig from its old cage to the new C and C cage you built. When you first put the guinea pig in the cage, let them explore and grow accustomed to their new surroundings for a day before interacting with them again.
Fold the walls up along the scores. Duct tape the corners of the coroplast together. Set the coroplast into the grid cage. Provide bedding and hiding places for your guinea pig. Introduce your guinea pig to its new cage.