Complete the following steps to learn how to shotgun jump.

Step 1. Go into a match and grab either a sawed off, or a snake shotgun.

Step 2. Move your mouse so it is under YOUR character.
(Once you have that you should be pointing the shotgun down)

Step 3. (There are two options)

Option 1: Once you have the shotgun pointing the ground, shoot the shotgun using Mouse 1 or Left click.

Option 2: Once you have the shotgun pointing the ground, jump and then shoot the shotgun using Mouse 1 or Left click as soon as you have reached max jump height.

If you followed the steps correctly, you should have jumped much higher than normal.

Try to position yourself over another player to deal damage while shotgun jumping.

That is how you shotgun jump. I hope this guide was helpful to you.