When you get your new fabric, before you do anything else, cut off a 8"x8" square and wash it and dry it in whatever way you are planning to wash and dry your costume. Check for shrinkage. Some rayons will disintegrate in the wash. One foil lame' print I tried washed just fine, but when I dried it, all the foil disappeared leaving an ugly blotchy gray on the black fabric. Unfortunately I didn't test it first; the skirt I made out of it was ruined.
Test for color fastness: Take a cup of hot water, dip the scrap ,trim, or beads into it, lay on white paper, and see if it bleeds color. Remember, most cottons from India are not color-fast. When you sweat, the dye can get on your skin and leave patches of color. My dance teacher once made a gorgeous costume of pink beads her mom had given her. She started to sweat and had long rivulets of pink running down her body and skirts. All those hours of work wasted too.