Probably the most popular item to upgrade on the road to serious sound, installing amplifiers is a much discussed topic. There's plenty of ways to do it, here's just one of them.
1. Match the gauge of your power run & ground cable.
2. Leave the power cables disconnected until you've finished connecting everything else! Power to the amps is the very last step before turning on the system.
3. Keep the fuse within 18" of the battery ! The fuse is there to prevent fires incase of an accident.
4. Keep the ground 18" or shorter.
5. Consider installing a new ground from the battery to the chassis (not shown in this video).
6. Ring terminals can be soldered, or beat them flat with a hammer.
7. Clean your ground point, even if using the seat bolts. You need a good metal on metal contact
8. The gains were set to the 10 o'clock position, which is usually a good match for a 4V headunit. Crossovers set to Pass Through as the headunits active crossovers are being used instead.