Yep, weld from the bottom up. Welding from top to bottom makes a pretty weld and its easier to do (you are not blocking your work with the mig gun), but you risk not getting good penetration. Slow the wire down a little or turn up the heat (just shy of burning through), and you won't have the puddle running all over.
Looks good so far. and take all the time you need to get it right. The few days or weeks that you save at this point will not seem worth it in when you're haveing to deal with some thing thats not set up correctly from the get go.
Brian Dunnigan