Clusters are not really complex things.
In a nutshell, there are Cluster Groups. You install SQL, and its resources get added to one of these groups that you specify. Then, through Cluster Administrator, you can set that group to run on either node (right click the group and click Move Group). That's essentially all there is to moving SQL instances between nodes.
The only difference between active/active and active/passive is that in active/passive, you have everything running on one node, while the other is just idle. In active/active, you have instances running on both nodes, but if one were to fail, it would be able to just move to the other node.
As far as installing SQL on a cluster..running through the setup is a bit different and there are a few extra requirements, but it's fairly simple once you go through it. There are a million pages out there with step by step screenshots (just google sql server 2008 cluster install).