Question for our metal fabricators!!!

[DOOM]

I would like to know the ''KEY'' to this type of weld. Is it metal thickness,welder settings, technique,mig ,tig ?????? I have really been struggling with this and I'm at wits end. I have to do some work to my outter wheel wells and I'm not at all happy with the final result. I need to get this dialed in so any pointers would really help. I used Jasons car (WAR) as an example of the type weld I'm struggling with..... My welders are Miller 210 mig Lincoln 110 mig I do have a new Hobart Tig but I have no Idea how to use it yet......