I never measure to thicken the stew liquid, it's all eyeball. It always comes out good. I would suggest starting with 1/4 cup of flour and about 1/2 cup of water, well mixed. I don't pour in the thickening flour water mix all at once, though, I start on one spot and add a little, stir it in, move about 90 degrees (compass-wise) around the pot and add some more. Depending on how those first couple of spots thicken I might give it all a good stir to let it stay with that amount, or continue moving around the pot adding more flour and water.

You could use cornstarch and water, but it doesn't remain thick; once cooled and then reheated the liquid is runny again.