Trimming a door jamb is a basic and easy task. All you need is the right tools and supplies, along with some good direction, and this makes for an easy DIY project.
Step 1 - Slice the Seams
The first thing you will need to do is to loosen the caulk that is holding the door jamb seams together. Take your utility knife, and cut into the caulk that lines the seams of the jamb.
Doing this will release the caulk, and allow you to easily pry off what you need to.
Step 2 - Remove It
Next you can take your pry bar as well as your hammer. Take the pry bar and and simply hit it gently with your hammer so that it slides in between the trim and the door. Make sure you only gently tap the hammer. Pounding on the pry bar will only damage your door.
To remove the door jamb completely, you will have to run the pry bar up and down the entire length of the door, while tugging the pry bar toward you the entire time. This will force the entire thing away from one another.
Again, don't forget to do all of this very gently and precisely. You can easily damage the area if you use force with any of these tools.
Step 3 - Make Sure All is Gone
There will probably be some caulk left behind that you will need to take care of. You will need to get rid of any nails and residue in addition to the existing caulk. You can do this by simply taking your utility knife and lifting these things off the area.
Step 4 - Mark It Out
Using your pencil on the side trim, you will want to mark the edges of it. This will occur at the corner of the opening of the jamb.
Make sure you make your markings dark enough you can easily spot.
Step 5 - Mark the Side Trim
Still doing some marking, you will want to take both pieces of your side trim and mark almost where the previous mark was. About one fourth inch above the existing mark is a good spot to be when marking your area.
Step 6 - Draw the Outline
Take your pencil for the last time, as well as your triangle square. Take the triangle square and put it on the trim to draw your line. Use your pencil to mark out a line that is dark enough that you can see without having to search for it.
This is the line you will follow when you are cutting in the next step.
Step 7 - Cut the Trim
Take out your wood saw. Using your wood saw very carefully, saw against the outline mark that you have just made.
There you have it. Easy to come by tools, and easy steps to follow! Good job!