Grey water systems are used to reclaim water that is still usable instead of sending it straight to the sewers. In this I will outline general guidelines for grey water and show a system that I have made with no pipe cutting required, which is legal (in theory) everywhere.

Step 1: Do Your Research

Before laying out a plan for your amazing grey water system, you must do some research. Learn the basics of grey water (I recommend greywater.com) and learn they grey water codes in your state (found at thegreywaterguide.com which as another useful learning guide) so that you're not seen breaking any laws.

Step 2: Layout Your System

Now that you are an expert on grey water, you can now start planning your awesome grey water system.

Every grey water system must have three parts; collection, a three-way valve, and distribution.

Collection (things that use water in your house and then have a drain):

Showers
Baths
Bathroom sinks
Washers (clothes not dishes)
NEVER use your kitchen sink, that is called black water and is not safe to use

3-way Valve (used to decide if the water is going through your system or to the sewer):

This is so if you use harmful chemicals they go to the sewer and not your soil
Required by law

Distribution (where the water goes):

Garden irrigation
Flushing toilets (not sure how legal that is, but it's a good concept)

How complex this is is up to you. Your system can be anything from having a five gallon bucket under you when you shower and then carrying it out to your garden to cutting pipe and running your own through filters and through your irrigation system.

Step 3: Build It!

Go live your grey water dreams, build your awesome system, and save some of that precious water.

The next several steps will be the system I have built.

Step 4: Introduction

The task at hand was to use the water produced by this shower on the second floor to water several garden beds at the back of the lot, about 500 ft away. We had no access to piping, which instantly made our job more challenging but more enticing. We made a system that mixes both the ease of the simple designs but the efficiency of the more complex systems. This system is easy to set up and fairly cost effective, so let's get to it.

Step 5: Materials

Something to stop water from going into the drain (plastic bag, or hand towel on top of the drain works)
A filter (coffee filters work just fine)
A pump
Method of moving the water (system of buckets, hose all the way to plants)
Something for people to stand on in the shower (totally optional)

Step 6: Setup

Collection:

Pump in the shower

3-way Valve:

The wash cloth

Distribution:

Hose to the garden

It's fairly simple, the wash cloth clogs the water and it pools at the bottom of the shower so that the pump can move the water to wherever you want it to go. The filter is needed to keep anything from clogging the filter. In our setup, we ran straight from the pump to a hose that ran all the way to the plants. If you are doing this, gravity is your best friend and use it to your advantage. Otherwise, the pump could go straight into a bucket with and overflow set up so that they flow into other buckets. Use all this collected water however you want (I don't recommend consuming it in any way), but we used ours to water our bamboo. If you leave it unfiltered, you should use it within a day or two (or use it right away if your state says you can't store it above ground).