There is a certain knack to FIM technique. I do it with my ffingernails rather than using scissors so that I can feel the central bulb of new growth. I gently press down the closest fully developed leaves to expose the tiny bulb. Then I take my forefinger and thumb and gently pinch it so that I can feel it, then I use my (cleaned) ffingernails to snip off about 2/3 of the bulb. It takes about 10 days after that to see the results.
However, if you have plants that are mature clones, you will not see the typical double cola that you get with non-mature plants grown from seed. If your plant only has 4-6 nodes with very little side branch development, you will not see much result for several weeks because there isn't enough branches available to take the opportunity to grow. If the FIM is done when you have several lower branches that are already developing then you will see very marked results as all off those lower branches will take off growing as the top heals and continues to grow out of the FIM point. But once this happens, you should see multiple colas developing around the point of the FIM after a couple weeks.