You have 2 types of routing loops.
The first loops the traffic and the other loops the routing table entries themselves.
A traffic loop is easy to do.
Just put in the same static route in 2 routers and point them to each other.
For example
If you have router A with ip address 192.168.1.1 and router b wiht ip address 192.168.1.2
In router A put in
IP route 10.10.10.0 255.255.255.0 192.168.1.2
in router b put in
ip route 10.10.10.0 255.255.255.0 192.168.1.1
now ping 10.10.10.1 from either router.
A routing loop of the routing table itself is very complex to simulate and very hard to fix.
Basically it is caused by route redistribution on multiple routers. A single routing protocol is smart enough to know that I do not want to learn a route that I just sent to another router. When you convert routes from one routing protocol to another some of this information is lost.
For this second kind of loop you need a good understatnding of administative distance. I do not have a simple example of how to do this since it in most cases will require 3 or 4 routers to get loop that will be of any interest to look at.
