For graduated size pearls use an inexpensive garage-sale-find costume necklace.
Because bargain jewels could turn out to be the real thing, make sure the pearls aren't genuine before separating them.
Cut out two pieces of fabric in any size you desire, to use as the pillow front and back. If you are stuffing the pillow with a manufactured pillow insert, use the measurement for it to determine the size of the fabric pieces. Iron the front and back pieces flat.
Draw the shape of the letter you will be monogramming on a piece of tissue paper, then pin the paper to the pillow front. Sew a large basting stitch in a contrasting color thread over the shape of the letter.
Sew each pearl onto the pillow backing, using the basting thread as a guide for placement.
When the pearls are all sewn on, remove any basting stitches and sew the pillow front to the back (right sides together), leaving a 5-inch opening to fill the pillow. Fill with batting or a pillow insert and sew the opening closed.