Loci are special places you will remember to check within your locus. (Your locus is a mental picture of a place you know well.)
You should explore your locus thoroughly before using it. Make sure you know your way around, recognize any special features, and look for places where you might be able to fit in a large influx of unusual items, should they happen along. These places are the loci.
Your loci become useful when you are required to remember a list of items. Move through your locus, placing items on your list on/in/around/under/through your loci. Make sure to visualize each clearly (The aardvark is in the fridge). Don't just stick it there, plan out the best way to get your aardvark into the fridge. Experiment. Then move onto your next loci/list item.
To retrieve list items, mentally walk through your locus, using the same path as you did the first time. Stop by every loci and check to see what's there. You should be able to find most of the stuff you placed there the first time.
Be sure to keep your trips through your locus chained together so that you don't get one list mixed up with another. Your locus will be affected by both retroactive interference and Proactive interference.
For a list of items that are hard to visualize, the keyword method may work better.