Acquire patterns for each style of garment you want. You may be able to alter a modern pattern or look for one in the costume section. You can use the same pattern for multiple garments and simply change the fabric to make it different. Refer to each pattern for the type of fabric to use and the amount of fabric needed. Since the character Viola dresses like her twin brother Sebastian, you will need identical clothing for both characters.
Pick out the fabric you want for each garment. Dresses look striking with simple floral or solids in jewel tones. Thick silk-like fabrics and taffeta will work well. Also choose notions and accents like ribbons or feathers. Men's suits will look nice with a solid fabric accented with shiny ribbon. Keep in mind the characters as well; you do not want a servant's dress to look nicer or more expensive than outfits worn by nobility. Olivia, who is mourning the death of her brother, needs to wear a veil; black organza will work well for this costume. Malvolio dresses in simple, Puritanical clothing; however, at the end of the play, Malvolio makes an entrance with bright yellow hose. Reference the play for any costuming requirements. You will likely also need notions like boning and zippers.
Lay out pattern pieces on your fabric and cut out the pieces for each garment.
Sew each garment by following the pattern's directions. For example, to sew a corset, you will sew the darts and stitch in the boning, finish the edges, and cover the boning with lining or ribbon.
Finish off each garment with trim like ribbons or piping.