Weight loss success is certainly to be celebrated, but it can leave you swimming in your old size 18s when you're ready for a 12. If you're still losing weight, replacing clothing may not seem like an option just yet, but you do need your clothes to fit properly and show off your new, slimmer shape. If you can sew, you may be able to alter some of your favourite bigger clothes to a smaller size and more flattering shape.
Choose the right garments to alter. Look for simple shapes and minimal seaming to allow for easy alteration. Opt for skirts, T-shirts, dresses and drawstring trousers; they're easier to alter than tailored slacks and jackets.
Try on the garment inside out, so you can see not only the fit but the seams. Stand in front of a mirror and start pinning. Work from top to bottom on your garment, dividing the excess fabric evenly among the seams.
Turn your size 18 right-side out and try it on again to assess the fit of the pinned alterations. Expect to remove and replace sleeves and waistbands, but try to keep button plackets, collars, and zippers away from the alterations---it will make for an easier process.
Set your sewing machine to a long stitch length and baste the changes into place. Pin to adjust the fit if necessary before you sew the alterations into place.
Sew seams with a short stitch length. Cut away excess fabric using pinking shears or trim seam allowances and finish with a zigzag stitch on your sewing machine.
Take tailored garments that you especially love to a good tailor for alterations. Try using your too-big clothing as fabric, arranging pattern pieces in your new size and cutting and constructing from there.
Some garments are just too big and alteration won't save them from an unflattering style or shape. If you never liked it, don't bother making it smaller.