Begin by entering a meditative state according to your preference. Once you are ready, visualize yourself at the edge of a forest. Fully picture everything around you, and yourself! What do the trees look like? How tall is the undergrowth? What are you wearing?
Once you've completely visualized the scene, walk into the forest. You are going in search of a body of water (be it a creek, stream, river... your choice). Walk until you find that body of water (for me, it's usually a semi-narrow stream).
When you reach your chosen body of water, study the current and figure out in which direction you can find the source of the water. Walk in that direction and continue to visualize everything around you as clearly as you can.
When you reach the source of the body of water, you should go ahead and get in the water at the source. Visualize yourself in the water. Are you going to swim? On your front or you back? Are you going to go inner tubing (my usual method!)? Picture it all in your meditation. Once that is set, go downstream, in the water.
Again, take note of your surroundings. Is the water moving quickly or slowly? Are there any obstacles in the body of water that you need to move around? If so, visualize and figure out how to get around them. Handle each one as it comes up.
Ride the body of water all the way back to the place where you first discovered it in the forest.
Re-emerge from the meditative state by whichever means you prefer.