IPs can be used many different ways. If you are just doing webhosting (not IRC hosting) then they are usually used for nameservers (possibly nameservers for your clients as well), SSL Certs, or just for somebody to feel special. Just because you have 5 does not mean you have to use more than 1.
Do you mean technically how you use it? or what are the possible uses of it?
Obviously the way to use it technically (i.e setting it up) depends on what use you put the IP into. i.e for an SSL setup on a certain domain will be different from using it as a nameserver for example.
15minuteservers/NAC gives you 8 usable IPs in addition to a "front-end IP" and yet another IP just for the control panel. My personal usage goes like this, since you're interested:
#1: Shared among sites that don't get their own IP
#2: Website for SEO reasons
#3: Different website for SEO reasons
#4: Different website for SEO reasons
#5: Website that uses SSL
#6: Website that uses SSL
#7: Nameserver 1
#8: Nameserver 2