Get the gear to build your anchor, including carabiners, locking carabiners, and a cordelette. You may also need webbing, cams(SLCD) and nuts. Ensure that you are familiar with their proper use before using as a part of your anchor. Much of this gear can be found online or in local outdoor equipment suppliers.
Setting the Anchor
A top rope anchor can be set using gear, existing anchors, or natural features. Or you may choose to use a combination of these options as well. With time and experience you will gain confidence in your ability to build a safe anchor, quickly and efficiently. Whether the anchor is built from gear, existing anchors, or natural features, it must be equalized. By equalizing the anchor you are allowing all of the gear in the system to share a part of the load, when holding a fall. It is my preference to do this by using a cordelette and a Figure Eight on a bight (refer to the drawing).
A Final Check
Although the anchor shown uses fixed anchors, you can replace any or all with gear or natural features. By building the anchor as shown you will distribute the load of a fall, as well as protect against the failure of any one of the three anchor points. To recap, some important things to remember: 1) Inspect your gear and do not use any equipment that is worn or not in proper working condition; 2) Existing anchors are always suspect, test the anchor and use your best judgment before relying on an existing piece of gear; 3) Ensure that your anchor extends past or over any edge, to prevent excessive rope wear or the potential of cutting the rope; and 4) Always equalize your anchor.
Read About It
There are many books available on climbing anchors that will provide you with additional information regarding the proper use of climbing gear and equipment.