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I melt butter, add flower, add the water obtained from boiling
macaroni (or similar), stir stir stir, add cheese. I always get those
disgusting lumps of cheese in the sauce. What am I doing wrong? Thank
you.

Try this , melt your butter, add the flour, cook it a little bit, stiring to
make your roux. Add some milk , Little by little, stirring all the time, the
mixture will be smooth if you cook it slowly and stir, then add your cheese,
grated, a little at a time, stitting all the time, . Will work out fine.
Rosie