Start by drawing a design onto a very thin piece of paper (tracing paper works great for this). OR print out the downloadable face design here onto thin paper.
Next cut around the design, leaving some space all the way around, but not too much that you'd have a bulky area to have to sew around. Then pin the paper to the clothing that you want to embroider, in the spot that you want the design to go. Again, this is another moment where using tracing paper for the design comes in handy because you can see through it to figure out the placement of the design, in relation to pockets, seams, etc.
Thread a needle with embroidery floss and then double or triple knot the end of a long piece of that floss (make it much longer than you think you'll need and double the width if you want a bold / thick line like you see in the photos). **See notes for beginner embrodiery stitch options.
Then start on the underside of the piece of clothing you're using, so the knot won't be visible, and hand sew the design through the clothing and paper that has your design on it, as shown in the photo.
Once the design is complete, tie a double or tripe knot on the inside of the garment once again to secure the remaining end. Then, you can rip off the paper by hand and it's ready to wear. Be careful though, you don't want to aggravate the embroidery floss design or cause it to fray / bunch up in any areas.