Before you even switch the welder on.. make sure fit up and cleanliness of the pieces to be joined is almost surgically clean..
degrease the area.. bright shiny metal.... tight fit of pieces..
Practice and develop your skills before performing the work that counts..
Pics of fit up of tubing at odd angles on a cage I built for one of my cars
If you're welding outside using shielding gas build a wind break or tent... stops the gas from being blown away
Argon/C02 75/25 is good for mild steel