A disclaimer: If your seats have airbags, heaters or other electrical accouterments, better move along on this one. Water and electricity don't mix. This is on the rear seats that I could take out without much difficulty. Think long and hard about doing this on an older vehicle in the salt belt as you could break off a bolt. Your seats and car could be sidelined a few days while they dry. Remove the seats from the car before you try this fix. As you're the one who has to decide whether or not to do this, you assume ALL responsibility for the outcome, come what may. Do this at your own risk. Do not use this method on leather or vinyl or I'll laugh at you personally. These are on cloth seats and only as a last resort.
My Buick Rendezvous is now rapidly approaching its 8th birthday, despite the 2004 model year horse feathers. It's also near the 150,000 mile mark and still going, poor wheel alignment notwithstanding (I can't redneck this one), but the interior looks like I've been carrying garbage in it for 16 months. The front seats I've covered (for now) but the back seats aren't much of an option. The light tan interior shows every stain, scuff etc for all posterity despite any method to clean them. I could attempt to recover them, but this is a spendy decision.
What you're going to need to do this are a few rags (NOT the red shop rags) a scrubbing brush, some pretreater (Resolve works great, but you can Shout it out) a spray bottle and some laundry detergent (Tide works best, but nothing with red dyes). You'll also need a hose and a nozzle. The spray bottle works best if you fill it nearly all the way with water first and then add about a teaspoon of laundry soap. If you add the soap first and then the water you'll end up with a yucky, bubbly mess in the bottle that won't clean anything and will be impossible to fill. Move the bottle around to mix, but don't shake it. Remove the seat or seats and spray the mixture over them. Just wet the whole seat down, but don't saturate the fabric. Too little is better than too much. You can add a little more to the stains, but easy does it as too much will be impossible to rinse off and make it easier to stain in the future.
Take your pretreater if you must, but do so lightly. Use a scrubbing brush to loosen the stain and rag to blot it up, or just use a rag on lighter fabrics. Again, don't work it too hard or you'll be recovering the seats. Once you're satisfied that every thing's done, get your hose and spray them down. If you see any dirt or in my case dyes from something the previous owners left, spray until the water runs clear. Do enough to make sure the soap is out and no suds remain. This can be done on a deck, but a cement driveway or patio works better because of the amount of water being used. Try your best not to spray water into the mechanisms or you'll have to oil or grease them later. If you have a few sunny days with low humidity, great. If not, better put them under an awning or in a garage to dry naturally under low humidity. If you have to put them in a confined space, a dehumidifier or room air conditioner is mandatory to prevent mold. I just set mine on my porch and will have to vacuum the leaves off of them.
Never put them back in the vehicle wet as you'll likely damage the electrical connections inside with the sudden burst of humidity. Once they're dry to the touch, use a rag with some WD-40 and wipe off the metal parts underneath. Put some Scotchgard on 'em and put back in the vehicle. You're done. Maranatha!