Building anchors is a complex topic with many variables that are beyond the scope of this website. However, understanding a few basic principles will give you a good foundation to build on and help you avoid some of the most heinous mistakes. With the information provided here you should be able to construct a position equalized and self-equalizing anchor given two fixed bolts using a cordelette or webbing. You should understand the pros and cons of each and be able to evaluate an anchor for rappelling.
All anchors whether they are for belaying, top roping, or rappelling should adhere to the concept of S.R.E.N.E.
Strong: Good anchor systems are built off of solid components, such as a good bolt, stout tree or immobile boulder.
Redundant: Anchor systems must be constructed of multiple components so that if any one component fails, the anchor will not fail.
Equalized: Building an anchor system so that the load is shared by all of the components decreases the chance that any one component will fail.
No Extension:If a component does fail, the anchor system should be constructed so that the remaining components are not shock loaded.
Using cord or webbing you can connect anchors together to equalize them and create redundancy, but also they are useful to adjust the length and position of the system so that the rope is not rubbing across the rock.