With a picture like this I would start out with the outline of the branches first using the sharp tip. You could then progress to parts of the squirrel that also need basic outlining like the feet, snout, and hairs around the periphery of the body.
Then take a pause and study your picture to formulate which areas you will start to shade first. You will want to lay down a very light shade first and will gradually build on that in various areas to build depth and give the picture a more 3 dimensional appearance.
In this case I started shading the branches and berries first and then moved on to the areas around the head and tail.
This is where your practice piece of wood also comes in handy. I will test my heat range for the lighter work and gradually increase the heat setting as I build the shadows darker. I also have a piece of fine sand paper glued to my scrap piece of wood. You want to periodically clean any residue that starts to accumulate on your sharp tip, so that it will glide through the wood with ease.
Later when the sharp tip has cooled I will run it on my leather strop to further clean and hone it back to its original glory ready for the next session.