A cow costume doesn't need to be complicated to be impressive. As long as people can instantly recognise that you are indeed dressed as a cow, you can consider your costume a success. Start with store-bought clothing to significantly reduce the amount of time and sewing necessary to complete your costume. Complete your cow look by chomping a large wad of bubblegum.
Cut random shapes out of black felt to create spots for your cow costume. Avoid cutting perfect circles because cow's spots are rarely perfectly round.
Glue the black cow spots to a white hooded sweatshirt and a white pair of sweatpants with fabric glue. Let the glue dry completely before you attempt to wear the costume.
Cut two ovals approximately six inches long and three inches wide from a piece of white felt. Pin one narrow end of each oval to the top of your sweatshirt hood. These felt ovals are now floppy cow ears.
Find a large pair of black socks and pull them on over your shoes while you are wearing the costume. This is much easier ifs the socks are at least one size too big for you. The black socks will make your feet look more like hooves.
Attach a large bell to a piece of yarn or a chain and wear it around your neck.
Wear black socks over your hands if you would like to create the appearance of having four hooves; but if you do, you will not be able to use your fingers well while wearing socks over them. Pin the felt spots onto the sweatshirt and sweat pants with safety pins if you would like to remove the spots after wearing the costume. You can use fabric paint to draw spots directly on the clothing instead of cutting the shapes out of felt but you will probably need several small bottles of fabric paint. Fabric markers will also work but avoid using a regular marker because it will bleed through the layers of clothing and could look messy.