I am now six months into a self imposed 'no new clothes for a year' project, but made this dress from a mixture of ingredients I already had.
Take 2 parts of fabric to one part each of 2 pairs of trousers, with a sprinkling of pillowcase.
There are more details and photos here
It might seem odd to make a dress out of trousers but one of my top charity shopping tips would be to look for the fabric you want in an unsuitable item of clothing. For example, you have an idea to customise a plain white top, but as plain white tops are easy to wear all the ones in the charity shops are well worn with crusty armpits. White skirts on the other hand, are not so easy to wear and are donated in good condition, so look to the skirts for your plain white top fabric.