Remember when working with basic slopers to draw patterns, that the outside lines of the sloper are the tightest fitting seams that will work for your doll. You added extra 'space' to the side of the shirt and the center front and waistline in the last step, now it is time to add space to the armhole.
The armhole marked on your sloper will fit tightly to the arm, and won't allow the sleeve to move or the arm to adjust position. To allow for 'ease' to let your arm move freely in your basic shirt, drop the bottom curve of the armhole lower than the one on your pattern sloper. See in the photo above how the top of the underarm seam allowance near the side of the armhole, is roughly the same as the line of the armhole on the sloper. When drawing the armhole lines, you add the seam allowance below the under side of the armhole, not above it, as you do for the other seam lines. This will create a deeper armhole when you sew the pattern and allow for ease of movement.
For a basic shirt, you want the side of the arm seams, and the top of the arm, to match roughly where the sloper touches the doll, allowing for the thickness of seams and the thickness of fabric, so your traced line outside the sloper works for thin fabrics, by drawing the pattern slightly larger.
When you have drawn the armholes on your shirt pattern front and back, use a length of thread or string to check the length of the seam lines that the top of your sleeve must fit into, by laying the thread along the shoulder seam line on the front of the pattern, then adding the length from the back of the pattern (without the seam lines at the shoulder or the underarm. Check that the total length is close to the seamline of your upper arm pattern. If it is too large on the front and back of the pattern, the width of your arm pattern may need adjustment. If it is too small, but not by much, you can 'ease' the sleeve into the armhole to fit, causing the sleeve to be slightly 'puffy' at the top and back of the arm. For the armhole on a doll shirt, one of the important criteria is that the doll's hand can fit through it easily.
When you have the basic shirt front pattern drawn out on paper, cut it free of the paper and transfer the seam lines and outside edges to a piece of non woven interfacing for a more permanent pattern. Mark the pattern on the center front edge and label it with the name of the doll it fits and the word 'shirt'. Check the interfacing pattern against the doll to check for fit, adjust the pattern if necessary.