There are quite a few quick or "R" strap instructables out there already. I looked at many of them but found them all lacking one feature or another I desired in my DSLR quick strap. So here is my offering.

Key features:

The ability for the camera to slide up and down the strap. This makes drawing the camera to your eye much easier.
A quick release for the camera. This makes for easy storage and tripod use.
A quick release for the body strap. This makes the rig easy to take off.
The ability to use this with my tripod adapter. This makes the camera ready to mount on the tripod.
**Bonus removable lens cap tether**

This quick strap works well enough that I removed my neck strap entirely and use the quick strap exclusively.

Step 1: Materials and Tools

Materials:

Six feet of 1" nylon strap (Walmart has these 'Sinch Straps' for $1.88 in the camping section)
2 Side release buckles, 1" wide ($.60 also in camping)
Bolt (1/4"dia coarse thread) or tripod adapter

Tools:

Cutting Utensil
Lighter
Sandpaper in a 1" strip

Step 2: Make the Camera Strap

We are going to start by making the short strap that will remain attached to the camera.

Cut off 6" of strap (about a pencil length).
Use your lighter to melt the cut edges so they don't fray.
Cut a 1/4" "X" about 1/2" from one end of the strap.
Pass the lighter around the hole to melt the frayed edges and push the bolt through while it's still warm.
Attach a slide buckle then the male side of a side release buckle to the other end of the camera strap Secure the tail with one of the slide buckles.
Attach to camera with bolt or tripod adaptor

I chose to attach the strap on my camera with the tripod adaptor so I can easily remove the camera from my quick strap and put it on the tripod, and vice versa.

Step 3: Make the Body Strap

Now we'll make the body strap, or the long strap that will go around your body.

Sand the inside of the strap channel on one of the female side release buckles so that it slides easily along the strap. This will involve sanding, then checking the fit, and repeating. You want to pay particular attention to any bumps or other grabbing/friction devices. Also, be careful not to sand too much off. You still want this buckle to be strong.
Slide the modified female side release buckle onto the strap.
Use the remaining side release buckle set to connect the ends of the strap.

Step 4: Put It On!

Put the loop over your head and right arm so the strap runs from your left shoulder to your right hip. Slide the buckle up to your shoulder and adjust the strap as necessary.

Attach the camera to the female side release buckle at your hip. The camera should hang securely where your hand rests. This makes it easy to hold when walking.

To take a picture simple grab the grip and bring the camera to your eye. The buckle attaching the camera should slide freely and make this a natural motion.

**Always check the entire strap , especially the buckles, before using to ensure it is secure. I don't anticipate the rig to come undone but there is no reason not to be careful.**

Step 5: Bonus Lens Cap Tether

Materials:

Lens cap
Crown bolt safety pin (spring type, I got mine at home depot)
Length of thin nylon chord (10"?)

Tools:

Lighter
Drill with bit slightly thicker than your chord

Assembly:

Melt the ends of your chord so they don't fray
Drill a hole in your lens cap near the edge
Pass one end of the chord through the hole and tie a knot on the end on the inside of the cap
(Optional: tie a second knot on the outside to secure the cap to the end, or put a drop of crazy glue on the knot on the inside)
Tie the free end of the chord to the safety pin.
Clip it to the camera strap of your quick strap rig between the slide buckle and side release buckle