The easiest way is to buy the roux blocks like this: https://www.amazon.com/Golden-Curry-Sauce-Mix-8-4-Ounce/dp/B0011UGYLM

Boil desired veg and meat and mix in blocks.

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The other way is to make the roux yourself and use curry powder.

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Nobody makes it 100% from scratch, at minimum they use the powder and thicken with roux or cornstarch.