There are a couple of considerations; physically making a connection, and protection.
Presumably your 8 gauge wire has a fuse or ckt breaker on the source end sized to blow or trip before that 8ga wire would be carrying so much current it gets hot, melts the insulation and causes a fire or sundry other problems. If you go to 18ga wire that can't carry the same current before it would get hot and cause similar problems, you want the appropriate fuse or ckt breaker at the transition (i.e. between the 8ga and 18ga wire) to limit the current in the 18ga wire to a safe value.
There are lots of options for physically doing it, from individual fuses to distribution blocks with fuses and breakers in them.