How to Use the knife tool

I am an average blender modeller and there is one thing that i have not yet grasped properly: using the knife tool. Whenever i want to cut, i first select the vertices, the use shift k, and try to cut but if i don’t know how to cut it properly. find an example here:http://www.freewebs.com/motorshow/Untitled-1.psd.jpg
what is the best way to cut properly? thanks

1.Select the edge to work on
2.Press Shift-k
3.Select Knife (Exact)
4.Click LMB to the right of the edge, then LMB to the right. You should see a yellow line.
5. Press enter

What do you mean by ‘properly’? I don’t understand the image.

actually from the picture, i was trying to cut the mouth of the fish. so i selected the two vertices (if i select one vertice, it says no vertices are selected), then did shift k, used the lmb to cut then i pressed enter. it seems to cut. so example i want to cut the mouth of a fish, do i use the knife?

My preference is the K key. It gives you three options for cutting and you only press one key. The knife cut limits the cut to the selected edges, but you get a lot of triangles in return. These could be problematic later on. For cutting out the mouth, you could try K, Face Loop Cut and click on any vertical edge in the middle portion of the fish. It would then produce a blue edge loop that you could move to the desired position. Left click to confirm. Then you could move the edge where the mouth is supposed to be inwards and you’d have a simple mouth for your fish.

thanks

Face loop cuts are definitely preferable to freehand knife cuts when possible, especially if you want to use sub-d later in the process. The cleaner the model, and the fewer triangles (as opposed to quads), the better your sub-d results will be.