The toughest part of making a gown is perfecting the bodice, this is what shapes the dress and, most importantly, what holds it up! I recommend finding a good commercial sewing pattern that explains how to use boning, bust cups, waist stay elastic, etc. Start by practicing a few times before cutting into your real fabric. Remember, a strapless dress has to be very tight to your chest and has to have enough structure in the top to hold up the skirt.