This is a brief summary of how I go about making the pine trees for my layout. It is fairly quick and easy. I tend to do a dozen or more at a time and would estimate that it takes 30-60 min/tree. But I've never really timed it. Usually what I do is identify an area on the layout that needs to be forested. I'll estimate the number of big trees needed to fill the area as well as the smaller understory trees....that's the batch I'll work on.
I start with a balsa stick. I forget where I got them, but it was online. They come in a wide variety of thicknesses. This stick is 3/4" so the trunk I will be carving in O scale with be a 3' width by 75' tall.
When I start carving, I start at the top of the tree. What I'm looking to achieve is a taper from ground to tip of tree. I whittle off the corners and start moving down. It doesn't have to be perfect, just get the taper and get rid of what looks like flat edges. These pics show the progression from stick to final carving.