Every good sand monument begins with a clear foundation of what you want the structure to look like. An architectural sketch gives a sense of the dimensions, and how much sand and space will be needed to create the project. It helps to visualize the castle or monument before you get digging. It will also help you determine if sand and water are the only materials you need, or if you will need additional tools for sculpting and decoration.
Once you've reviewed your sketch, dig an appropriately sized mound of sand. Then decide which method to use to form the shape of your monument: "softpacking" or "handstacking," as described at Sand Castle Central (sandcastlecentral.com). Softpacking involves working with damp, but not dripping, sand. Handstacking requires that you build at a point on the shoreline that is close enough to the water that you can easily draw sopping wet sand from a hole, but far enough away that your project will not get washed out. A third option is to use forms, or large open-ended cylinders designed specifically for this type of building. You can fill the forms with sand then remove them to reveal an efficient and solid base for your structure.
You can use your hands to define the overall form of your monument. If the shape is not particularly defined, using your hands will likely be sufficient for this task. However, if you make something involving faces or distinct traits, you may want to use tools such as artist's trowels or serrated blades to cut definition into the structure.
No sand monument lasts forever, even the most strategically located one. Wind and rain will eventually wear the structure down. But you can prolong this end by taking measures such as positioning it a safe distance from the water in both high and low tides. Using a spray bottle to keep the sand moist and firm will also keep it from crumbling quickly.