How do I introduce a new guinea pig to my current one?

Short answer: quarantine for about two weeks, then hold short meetings on neutral ground, and only house them together once things are calm.

1. Quarantine — separate cage for two weeks plus a vet check; let them sniff through bars.

2. Neutral space — meet in a pen neither has used, with two hay piles and two hideouts.

3. Normal behaviour — sniffing, chasing, rumbling and head-raising sort out rank.

4. Separate immediately — raised fur with teeth chattering standoffs or real biting; use a towel.

5. Before moving in — deep-clean the cage and use fresh bedding to remove territory scents.