Nutribullet works really well. Also recommend mixing in advance and leaving it in the refrigerator for several hours/overnight to thicken.
I actually found that the shaker that came with my first order worked very well for removing lumps - if the US one is the same as the UK version, the shaker is one of the ones with a plastic grid in the lid that breaks up lumps as you shake. Far more effective than one of those metal shaker balls. Only real tips are make sure you put water in first, then Huel, otherwise it all gets stuck at the bottom. Also try to avoid letting the remains of your shake dry in the bottle/shaker. The oats tend to set like concrete, so always rinse out everything ASAP once you've finished your shake.
Quantities-wise, that's really a trial and error thing. I like my shakes quite thick and gloopy, so I used less water than the directions suggested (I can't remember how much exactly as it's been so long since I had Huel in the UK), but you'll soon find a ratio you like.