Covered buttons with delicate lowercase letters (left, top) are embroidered in a satin stitch. Felt patches with satin-stitched initials (left, middle) are tacked to the slippers with French knots. Calligraphic letters embroidered in French knots (left, bottom) have a raised, nubby look. You can also make a monogram without embroidery by tracing the outline of a letter onto felt, cutting it out, and gluing or hand-stitching it to the slipper.
If you're new to embroidery, follow these techniques for the satin stitch and French knot:
At an angle or straight across, these side-by-side stitches fill in the outlines for a shape. Pull the needle through to front at 1. Insert at 2; pull needle back through at 3, right next to 1. Keep stitches tight and flat for a smooth finish.
These are useful for creating a raised point of interest. Pull needle from back of fabric to front at 1. Holding thread taut with one hand, wrap it twice around needle; insert needle again close to 1, keeping thread taut as needle is pulled through to back.