Ahem...with that said

how do you felt, again?
lol

Put a low level of hot water in the washing machine with a tiny bit of detergent. Put your bag in a net bag or a pillow case that’s tied and wash it with something else–jeans are good–to increase agitation. Check it every 10 minutes to keep an eye on it’s progress. If the water gets cool, drain it and refill with hot. It can take anywhere from 10 minutes to an hour or more, depending on your yarn.

When it’s the size you want, block it on something to hold its shape.

do the colors run and that is why you need it in a bag?

No, the bag is prevent it from snagging on anything (jeans button or zipper, washer parts, etc). Also if you’re felting more than one item, it keeps them from felting together. The bag is basically to keep your hard work safe and secure during all that agitation.

Also, the bag prevents the excess fabric that comes off your felted object from clogging up your washing machine’s water pump. All those little bits of wool that have been shed can really cause some problems.