I've ended up following the instructions and it going lumpy before but the last couple of times it's turned out fine. I used a hand whisk to mix it before putting into the microwave, microwaved for a minute at a lower wattage (750, not 900 which is the top wattage on mine). Stirred and whisked it again after a minute, making sure that any lumps forming are blended into the liquid. Put it back in for a minute and then stirred like mad when finished to mix it properly. Leave for a couple of minutes to stand before eating.
I used exactly the amount of milk stated (otherwise it was too watery).
I tend to just guess with mine. I don't have skimmed milk in the house as my lads only have blue milk so I water that down slosh it in a bowl and sling it in the microwave and keep stopping it to stir it up, probably for 1.30 or 2 minutes altogether.