I'm just ballparking here, but 1 1/4 cup sounds about right.
Basic starter directions:
Boil about 2L water (you'll lose some to evaporation so you'll get close to the 1.8L you're shooting for by the end of the boiling), add the DME (and 1 tsp yeast nutrient if you have some), continue boiling for 10 min or so, then put in an ice bath (or just cool it) until it's 80F. Sanitize the starter container and anything else that'll come into contact with anything after this point. Put the wort into the starter container, then pitch the yeast into that. Aerate REALLY well - and also swirl the solution as often as you can to keep aerating the wort during the starter's fermentation ("every time you walk by").
Then you need to either:
1) try to pitch it into the wort at the high krausen (but it's really hard to judge that - could be 12 to 24 hours or so) - you can always just ballpark it, and pitch it when you're done brewing and your wort is ready OR
2) let it ferment out for a couple of days until it's beginning to clear and at that point put it into the fridge. Pull it out when you start your brew day so it can get to room temp. Decant off a bunch of the clear liquid (but not all), swirl the yeast into the remaining clear solution, then pitch the yeast starter into the fermenter when the wort's at fermentation temp.