One newbie mistake here is not backing off on the amperage as you end to prevent cratering, and not allowing the proper amount of post flow. Post flow is the amount of shielding gas that continues to flow after you’ve finished welding. This flow prevents contamination and oxidation of the weld puddle as it solidifies. The amount of time depends upon the thickness of the metal as well as type, but around 5-7 seconds is a good starting point. This results in a structurally stronger weld that doesn’t have a potential weak spot between two weld beads. For a corner joint like this, both edges of the adjoining pieces should be melted and the weld pool should be kept on the joint centerline. You’ll also need a bit more filler rod to create a nice convex bead to suitably join the two.