You'll manage with a sponge-down for a day or two. I'm sure Sqad will be along shortly but having lifted a cast resin shower base yesterday I'll give you my recommendation: 400mg ibuprofen now, and 500 mg paracetamol at bedtime, lots of fluid to take the tablets down.
Rest on back on the floor with knees bent up to ceiling, same when you retire to bed.(as at 4.50 on the video below). You'll probably shift position when you nod off but this is a good position for bad backs.
See how you go in the morning, keep the cold compresses going.
Try gentle back stretches as you go along, do nothing that hurts.
http://www.nhs.uk/video/Pages/sciatica-herniated-disc.aspx