Before you even consider crosscutting your material to length you should inspect and calibrate your miter gauge as necessary. 

Most manufacturers advise you to use a square against the blade and calibrate in this manner, but I've found that this is not sufficient in my experiences. Aside from buying a calibration jig such as a Miter-Set or similar, the 4 or 6 sided frame method is my go to for calibration. It works as you would expect:

1. Set your gauge to 45 or 30 degrees depending on which you're cutting.
2. Use a stop block to ensure all pieces are the same length.
3. Tape up the frame as if you were doing a glue up.
4. Any discrepancy in your miter gauge will show in a gap. Adjust the calibration in or out depending if you have a gap in the inside or outside of the joint.

For more exposition on this, there are copious amounts of video on youtube that go into other methods of calibration on miter gauges or crosscut sleds.

I took my rough stock and milled to a final square. I made sure to mark two sides so I could keep track of which sides I would plane in pairs to retain a square profile. 

I chose to use an 8 degree angle for the sides. This was an arbitrary choice. For a larger bench you could go with something more severe, but I scaled it back enough so I could still have a noticeable trapezoidal shape, but not have the legs come to a point. 

To keep legs in pairs, I used a carpenter's triangle on each side pair and the top and bottom pairs.

I tried to do all operations in the same go. I cut the bottoms first, then set up the stop block for the tops and so on. 

To make sure you don't get turned around, use a pencil to mark the cut directions so you'll always have a reference when you take each piece to the blade.

Here you can see my stop block to keep all 4 vertical members identical in length. 

Moving on to top and bottom pieces. Note the use of a carpenter's triangle and bevel marks to keep me honest.

After cutting the bottom to final width, I clamped the bottom joints taking care they were not skewed and set out to mark the length of the top.

I exaggerated my knife line here with my white pencil (works great for walnut btw). I used this on almost every cut I had to make based on a reference mark.

Note that I'm approaching the blade from behind. This way you see both the carbide and your knife line for easy alignment. This assumes your blade is fully parallel to the miter slot.

I layed up the legs in this orientation to double check that all my angles were cut correctly.

Much like I did with the slats, I used the cut and flip method to check for my half lap depth. Because I had some off cuts, I made pairs that I could nest to check my final result with.

They mesh perfectly (on the outside faces), which means my blade height is exactly where I want it. I then locked it down and didn't touch it until I was complete with all 8 joints.

Here I began to mark the areas I had to remove and set a marking gauge to the thickness of the leg stretcher so I could properly set a stop block for each joint.

First joint turned out really well, lets hope the trend continues!

Tight seams on all sides. I purposely let the end grain stand proud a 32nd of an inch so I could plane it flush after glue up.

Here is where the tough part starts if you don't have a miter gauge like an incra 1000HD. You'll only be able to cut half the half lap joinery with the gauge set where it is. To cut the other side of each leg, you'll need to swing the fence to the 8 degree on the opposite side. Gauges with detents on every angle like the HD make this easy. I had to make sure that my fence was at the exact same setting by eye for the second set on each leg assembly.

With the gauge re-set, I utilized one of my offcuts and cut a new (deeper) cut in the same angle orientation. Here, you'd be able to see if your angles are wildly off from one another. For more precise inspections, a set of calipers would work. You would just make sure the distance between the two cuts is equal from one side to the other.

The way I knew I had the correct angle was resting the 'new' cut in one of my existing half laps. The angles nested, and in this configuration, if there was a discrepancy in the angles, the vertical portion would not be flush with the leg joint. 

You can see here I have a flush and tight joint along both fronts. It's time to cut the second set.

The next obstacle is making sure I have the same measurement to my stop block. I got lucky and nailed it on my first try. 

I got a sanity check by taking both legs and resting them up against a registration block. If the kerf is continuous across both you're good to go.

Here is the money maker on these legs. Although you have measurements in plans, it's all about cutting to fit. I could have clamped the bottom joint and had the vertical ride up to the uncut top joint, but I wanted to avoid that unnecessary angling of the leg. So I clamped the stretcher to the INSIDE of the normal placement and used this as a guide to make a scribe on the top for the shoulder. 

Had I done this the other way, I wouldn't have such a clean line on top to scribe with. There would also be a chance to introduce error when flexing the stretcher on the uncut joint.

Make sure the bottom joint is flush with the shoulder and tight across the cheek. Any error in setup here will translate to your top member.

I used this clamped setup to make sure that my top piece came out to the right measurement. If it was a squeeze getting it in with the bottoms clamped, it meant I needed to remove a hair more. If it was loose I would have had to trim the bottom.

Dry fit on a flat surface.

Looking great!