Fairly straight forward from the diagram, first you want to cut a hole near the bottom of the bucket and attach the connector (using sealant). Ensure it is water tight by filling the bucket. Next, cut your garden hose in half; connect the hose connector on one end, then stick the tap on the other. Connect the 2nd half of hose on the remaining end of the tap and then attach to the bucket. Use sealant or glue where necessary!
Next drill two 4mm holes (or equivalent to the size of elbow joints), insert elbow joints and seal/glue. Then cut an appropriate length of air tube to connect the joints.
You have now created a water depth gauge! You could insert a little red float bead in the air tube too, but I find that the water level is quite visible as is.