Painting reflections is not as difficult as many new painters think. In order to do it properly, you need first to understand that you are painting a distorted version of the environment of the reflector. In a three-dimensional world, determining the perspective of the reflectors and whether the reflected surface is rough or smooth is of great importance. Here are a few useful guidelines to follow:

The rougher the surface the more distorted the reflection will be.
The rougher the distortion the rougher reflection.
When working with objects like glass, keep in mind that you should not only paint the mirror image of the object being reflected but also the distortion of the smoothness of the glass and the curve of the reflected image.

In the image below you can see that the rock or potato is showing on the glass where the surface is concave and elongated. (Credit: painting-techniques.net)

To truly understand how to paint all reflections on canvas you must understand the three components listed here.

The angle of the reflective surface.
The smoothness of the reflective surface.
The shape of the reflective surface.

I would recommend that you start learning with a water reflection and then continue on to the glass or object based surfaces. Always work with a still life setup or a picture. Also practice with charcoal and paper focusing on darks and lights as shadows and highlights then you can upgrade to color with acrylic and on to oils.

How To Paint Water Reflections

Genera Tips

Dark colors in dryland become lighter when reflected in the water
Light colors on dry land become darker when reflected in the water.
The colors of the sky appear slightly darker on the surface of the water.
All objects when reflected in the water appear slightly darker and dimmer.

Water Becomes Less Reflective the Closer the Viewer Is:

Water does reflect perfectly the sky and the surrounding nature and objects.

The distance of the viewer to the water affects how clearly the image of the earth and sky are reflected.

When painting objects next to the water, the shadow cast by these objects usually makes it easier to view bellow the surface, as the object blocks the reflection of the sky.

When close to calm water the viewer will only see a weak reflection directly below him/her, while the outlying water will refect details, acting nearly like a mirror.

When observing dirty water from a close distance you will be able to see the overall color of the water itself.

When the water is very clear you can see the rocks and objects under the surface.

Understanding the Shapes & Angles Reflected in the Water:

One mistake that many new painters do is that they paint the reflected images exactly as a mirrored object. This is not a good practice, as a realistic painting requires a more accurate representation of what is being observed. Here are a few things to keep in mind:

The reflected scene in the water is the actual object reflected from two different angles.
The reflected image in the water bounces off the surface of the water.
What you actually see is the reflected scene bellow the surface from an angle for as far as your eyes can see.
You are actually seeing more bellow the reflected objects and even reflection of objects that you can not see at all when you look directly at them.

Watch the following great video to lean how to paint reflections:

How to Paint a Reflection of a Sunset

If you are capable of going under the ocean, you will see a different world. You will find everything floating in still motion. There are bending rays of sunshine refracted in the water and offering a variety of hues. Is it not an unforgettable scene?

If you are a painter and have skill and expertise in painting the scenes above the water, you will still find it challenging to capture the versatility and beauty under the ocean. It needs patience and extraordinary skills to go into the water and the capture real looking scenery. You should have right tools mingled with exceptional skills for the purpose. You need to be slow and steady. You should demonstrate your experience of making the masterpieces of arts. Only then you may have pride in calling them your creation.

You need to make sketches of your work on a surface. It may be a wall, canvas and a paper that may feel you better. It should be remembered that there are no strong spells of gravity as it on the earth. It may stretch your shapes, and you may twist them in a way that may not be possible on the earth.

Tips and techniques

There are some techniques for this venture. Here are some tips that may make your sketches amazing and excellent.

Background

Create blue shades for the background of your image.
Mix three blue light and dark shades that may offer a distinct view.
Paint the lightest colour on the top to make the shining light prevalent.
Moving down the painting make it darker gradually.
It is better to provide a background colour to picture.
On the meeting points of colours strike the brush to mix the colours properly
Transitions should be mild from light to darker.
Before starting your background should dry up.

Items on the background

You may paint different items on the background. It may be a coral or sand.

You need to use a tan colour for sand.
You may choose a customized colour for coral or fish.
Bright orange colour fish and grey colour coral may be the best background item.
All should be in a match with the rest of the image.
After the background is dried up, you may add strips and shadows.
First, paint the main object to avoid obscurity.

Focus of image

Now you may paint the focus of your painting. You may take a diver, sunken skip or a whale as your focus point
The focus colour should be vibrant.
Highlights and shadows should be on the focus images.
After primary colours dry, you may add smaller objects.
You should hide the whole painting in the light blue transparent glaze. It will provide extra underwater effects.
Spay on clear cot should be placed on image for durability.
Acrylic paint is best as it dries quickly.
You may hide your mistakes early with acrylic paint.

How to Paint a Reflection of a Sunset