Good question!
Use a yeast calculator, like BrewUnited's or Yeast Calc, easy to calculate step starters, or starters made in other ways.
Looks like you can do it in one step if your yeast is fairly fresh, say less than 1 or 2 months old.
Pitch the whole pack of 200 billion cells to make 2 liters of 1.040 starter wort. Since your flask volume is limited, include your yeast slurry volume (weigh the pack) in the starter wort calculation. I.E., you can probably use 1.8 liters of 1.044 wort and add the 200 ml yeast slurry (or how much is actually in there) from the pack. That will result in 2 liters at 1.040. Higher gravities for starters are not recommended, but if you need the extra cells, go for it. I doubt a 1.050 starter won't be as good as a 1.040 or 1.037 one at the end of the day.
That said, I've made step starters for larger pitches by piecemealing from smaller starter vessels. Like using two 2 liter flasks. Either at the same time or one after the other. Cold crash in a separate vessel, like a gallon or half gallon jug, or my favorite, 1/2 gallon pickle jars, as you only want to pitch the slurry not the beer on top.
I get the best and fastest growth by oxygenating my starters for 2-4 minutes at 1/16 l/min. They get very foamy, and I let them sit like that for 4-6 hours before stirring/shaking, as I think the agitation knocks the O2 out of solution.