How does Napoleon show his disapproval of Snowball's plan to build the windmill in Animal Farm?
I assume that you are talking about what happens in Chapter 5. In general, Napoleon disapproves of everything that Snowball does. He shows his disapproval in various ways. But the most surprising way in which he does this comes when he looks at the blueprints of the windmill. What he does to show his disapproval is to lift his leg and urinate on the plans.
Up until that point, Napoleon had just talked about how Snowball's plans were dumb. He had never actually looked at them. But this time, he looks at them and urinates on them, which I suppose is a pretty effective way of showing disapproval.
In Ch.5 we read how Snowball with a lot of painstaking effort worked out the details of the plan for his windmill:
Snowball used as his study a shed which had once been used for incubators and had a smooth wooden floor, suitable for drawing on. He was closeted there for hours at a time. With his books held open by a stone, and with a piece of chalk gripped between the knuckles of his trotter, he would move rapidly to and fro, drawing in line after line and uttering little whimpers of excitement. Gradually the plans grew into a complicated mass of cranks and cog-wheels, covering more than half the floor,which the other animals found completely unintelligible but very impressive.
However, Napoleon who was jealous of Snowball and whom he had always considered to be a threat to his position disapproved of Snowball's plan to build the windmill:
Only Napoleon held aloof. He had declared himself against the windmill from the start.
He expressed his disapproval in an obnoxious manner by urinating over Snowball's intricately drawn out plans for the windmill:
One day, however, he [Snowball] arrived unexpectedly to examine the plans. He walked heavily round the shed, looked closely at every detail of the plans and snuffed at them once or twice, then stood for a little while contemplating them out of the corner of his eye; then suddenly he lifted his leg, urinated over the plans, and walked out without uttering a word.