I can tell you the way I make paste, and it always seems to turn out well and not have any lumps.

I heat a large pot of water on the stove until it's simmering.
Then, I hold a wisk in my right hand and a seive in my left hand over the pot, and I pour about 2 -4 cups of flour into the seive. As the flour falls through the seive into the water, I wisk the water quickly to mix it in.
I just shake the seive every few seconds and continue wisking the water until it's all mixed in.
Then I leave the pot simmering for about 1 hour but I mix it a few times with a wooden spoon.
(I don't use salt.)
Then, I check the consistency which should be like gravy.
I leave the pot for another hour or so to cool, then I use it.

It's kind of a long process but always satisfying, and the paste is nice and warm on the hands.
It will last for a couple of days, before going moldy.

Happy paper macheing.