After years of preparing velouté for Dutch croquettes, I think I can explain how to prepare this sauce without getting lumps. The secret is quite simple: don't mix in the liquid with the heat on. Having said that, here's the method. Heat the liquid. Milk for bechamel. Fond for velouté. Prepare a roux. Melt butter and add flour. Normally, the proportion is 1:1, but I use 1:1,5. The flour is the thickening agent. So, more flour - more thick. For a small batch, use 100g butter, 150g flour.Cook the roux thoroughly on a medium high flame. The roux will change colour and you'll see it becoming clearer. It will also start smelling nutty. Kill the heat and add the hot liquid slowly (in increments) while stirring with a ladle or whisk. Some prefer the whisk, I prefer the ladle. After adding all the liquid, turn the heat back on. Keep stirring and finish the sauce. Clumps will only form if you have the heat on and you don't stir.