As a sewing newbie, I tried to make this as simple for myself as possible. I just took some excess cloth and a pretty clasp I bought from Joann's and sewed away without any planning... There's no buttons or zippers, just straps and a clasp :]
Take the fabric and cut out 2 rectangular pieces. To calculate the WIDTH, measure the widest part of your body (the hips) and add 2~4 inches, depending on how snugly you want the dress to fit. To calculate the HEIGHT, measure from the top of your bra to where you want the dress to hit you on the legs and add 3 ½ inches.
Flip the dress inside out and it should look like a really long tube top. Slip it on. Gather the excess fabric on top and bunch it together so it fits snugly across the top of your bust. Now take the connected clasp and hold it over the two flaps you've created. Use a fabric marker to mark where each half of the clasp should go. Let go and take off the dress. Notice the marks look farther apart on the fabric than when you had bunched it together. Hand-sew on the clasp.
Put the dress on and clasp the clasp. To determine the LENGTH of the straps, measure from the top of the front of the dress to the top of the back and add 2 inches. To determine the WIDTH of the straps, choose a width then add 1 inch. Cut out two pieces of fabric with these measurements. Flip the two pieces so the wrong side of the fabric is facing you. Fold in the left and ride sides of each piece to the center then fold the whole thing in half. Pin, iron then sew along the side where all the overlapping fabric is. Finally, stand in front of the mirror and arrange the straps where you want them to be and pin them down. Take off the dress and sew on the straps along the pins. Secure by sewing over the straps several times, and tada, you're done! :]
*** VARIATIONS YOU CAN DO
A) Stop at Step 6 and leave it a strapless dress. Great for the summer!
B) In Step 6, instead of sewing on the clasp if you don't have one, cut holes in the marked places and thread a ribbon through the holes. Tie a bow with the ribbon in front to secure the dress.