Paint clean water on the bottom half of the painting for a wet-on-wet technique. Let it soak in so it is just damp. Make sure it is not too wet or too dry.
Add some ultramarine blue, starting in the foreground and then blending it back towards the trees.
While this blue area is still wet, paint a mirror image of the trees from up above. When it dries, the colors will blend and look like a reflection.
If desired, leave a few horizontal gaps without color in the water to make it look like there are ripples in the water. Alternatively, when it is dry you can lift out little horizontal lines. Leaving the water smooth is fine too.
Paint in the shadows and highlights in the reflection in the same manner as above.
Using loose strokes, add green paint to the middle part of the tree reflections. Look at your painting and try to mirror the same darks and lights in the water.
When your painting dries it should all blend into a nice soft reflection. If you want it to be a bit more blurry after it is completely dry, gently brush on some clean water and let it dry again.