Once you select the sounds you want to use, create the patterns, and melodies, you're ready to apply effects to fix or enhance the sound from each instrument or sample.
1). Go to Step Sequencer view. As explained earlier, you can toggle between views with these set of buttons inside the top toolbar. Step Sequencer view is button #2 here.
2). Select a track you would like to apply an effect to.
3). Go to Mixer view (button #5 above).
4). Right click on one of the Insert columns (channels). 
5). In the pop-up menu, select 'Link selected channels > to this track'
6). With this channel still selected, go to the right column where it says IN, and click on the first drop down arrow. This will give you a list of available effects you can apply.
In this picture, Fruity Parametric EQ 2 effect has been applied to channel 2.
7). Apply as many additional effects as you'd like, by adding them to the other slots in the column on the right.
FL Studio applies effects in the order from the highest slot to the lowest. The sound will be processed through the effect in the first slot first, and than through the other effect in the slots going down.
Each slot has a green light next to it. Toggle it off, to disable processing of a particular effect.
Each slot also has a knob next to that green light. Move it to the left, to apply an effect partially.
9). Below the effect slots, is an effect settings area. When you select a particular effect, this area will contain parameters you can adjust for that effect.