Inside, warm bathroom, warm water, baby, bird or dog shampoo. Have towels and a blow dryer ready & waiting. Vinegar and glycerin are helpful.
Fill a small tub/bucket (about the size of a dishpan) with toasty warm water, add a large splash of vinegar and shampoo. Gently place your bird in the water, which should at most come up to her chest. Make sure you have a firm hold of her and especially that she cannot get her head under water at all. Swish her around to get the soapy water through her feathers. Use your fingers to gently lather her, and to work loose any clumps of dirt or manure. Be careful to not pull feathers. Once she is nicely wet, remove her from the tub and work additional shampoo through her feathers. Be sure to get her crest, vent and feet clean. Swish her through the tub as necessary, and replace the tub with clean water as necessary. Rinse under the faucet like a shower, being sure to keep her beak pointed upwards and out of the water. Once you have most of the shampoo off, refill the tub with clean water and another splash of vinegar plus a splash of glycerin. Swish her through the water until she is shampoo-free. Wrap in a towel. After a few minutes trim her nails and beak if needed.
Unwrap the towel and begin blow drying--keep your hand between you and her--if your hand gets hot, lower the heat. With most breeds you want to blow in the direction the feathers grow. With silkies you blow in the opposite direction. Work through all her feathers a section at a time, the feathers on the back & under the wings tend to clump together, so gently separate them as you dry. Once she is fairly dry put her in a cage with a heat lamp. As with chicks, offset the heat lamp so she can choose to be warmer or cooler.
Keep her inside for a good 24 hours after she is completely dry; longer if your weather is cold. While she is inside is a good time to check her droppings & worm as needed.