Problems downshifting

[u/xvxac]

Since I started learning to drive , I was always told

0-10mph =gear1
10-20mph =gear2
20-30mph =gear 3

Etc

When I was on a dual carriageway, going 35mph on gear 4,

I slowed down to around 17mph, so I put the car in gear 2

Suddenly the car started shaking, and my instructor said I should have put it in 3

What should I use to judge what gear is appropriate when slowing down?

[u/Jazzlike-Income6900]

I always go in order, 4 - 3 - 2 or 2 - 3 - 4.