i made one for my friend once.
i took the measurement of her thigh and cut a length of 1" wide elastic slightly shorter (i think i was about 1/2", but whatever's the most comfortable for the person wearing it and remember seam allowance).
then i took 2 lengths of 1 1/2" grosgrain ribbon twice the length of the elastic and sewed the top and bottom of the ribbon.
after the ribbon tube was finished, i threaded the elastic in (using the good ol' safety pin as needle trick!) and then sewed the ends of the elastic together. make sure you sew it together very well, you don't want the elastic to pop apart and the garter to drop to the floor!
then i folded the rough edges of the ribbon in and sewed them together. i put the decoration where the seam was so that the whole thing looked nice and neat!
as for decoration, you can do what you want, use your favorite things, colors. . .
my friend, rebecca, VERY handy (her nickname's tool girl) so i made a little glittery hammer and yellow tape measure with her and her hubby-to-be's initials on it. i had ribbon coming out of the tape measure, like tape and then wrapped that around the medallion using little flowers that matched her dress.
here's a not so great picture of it (but it's the only one i have):