This is a desktop I am making which I want to add a breadboard end. Breadboard ends do 2 things. They hide end grain, and they can help keep a panel flat. However simply attaching a piece of wood to the end of a board will lead to disaster. It's very important to let the large panel move. A proper breadboard will control the movement, but won't restrict it. 
Here I have the panel and the breadboard, with the tenon laid out
The general rule is the tenon length should be 2/3 of the width of the breadboard. My breadboard end is 3" wide, so my tenon is laid out for 2" deep.

I use the breadboard itself as a batten to guide my saw to cut the shoulders. I have it clamped to the board right on the gauge line. I have a bit more hanging off the edge where I start the cut to help start true. 

Now I saw down my knife line with the breadboard as a guide. I take full strokes down the board until I hit the line I marked for the depth on both edges. I've found that the middle 3rd is often still a little high, so I take a few extra strokes in the center after I am down to my lines on the edges.

Flip and repeat. Now the shoulders are established and they should be mostly straight and plumb.

Now to remove the waste. I clamp the breadboard back in place right over the saw line to avoid breaking out the fibers in the table top. Then I use a 1" chisel to carefully split some of the waste 

With the chisel angled up slightly, I start splitting out some material. Do not get greedy here. I take a light cut at first because some pieces of wood will split really easily and I can go right past my baseline. Once I get a sense for how the wood is splitting, I go a bit heavier. I do this until I am about halfway to my depth gauge line

Then I clean it up with my router plane. I have an auxiliary base attached to I can keep a good portion of it registered to the table top even when cleaning up the leading edge. Like with splitting, I take lighter cuts at first to see how the wood behaves. This is a cross grain cut and tearing/splitting could be an issue if I go too heavy (and it was on the board in the center).
I continue this until I am at my gauge lines  

Now I trim the shoulder to make sure it's square. I don't want to move it back at all, I just want to be sure it's vertical or even very slightly undercut so the breadboard meets with no seam. 
Once complete, flip and repeat

Now to lay out for the tenons. I am doing a blind breadboard meaning none of the joinery will be exposed. As such I remove 1/2" of the tenon from each end
I leave 3 full length tenons. One in the center, 1 on each edge. Why? To allow for a strong joint, but to reduce some surface area to allow for proper wood movement. I mark out the long tenons and leave the rest at about 3/4" long 

I then saw the long tenons to my 3/4" baseline mark. Absolute precision is not needed here because these will not be tight fitting mortises  - but try to keep it straight 

I then saw off my shoulders but not right to the baseline. I will clean that up with a chisel after to ensure I don't cut into my table top and the edges are straight and plumb 

With the tenons marked and sawed, I am going to remove the waste with my turning saw, a coping saw would work as well 

I start in from one side and then saw along the baseline. Once done I turn the blade the other way to clean up the waste that is left behind. Like with sawing the tenons, absolute precision is not needed here. Some will come in after and clean up the baseline with a chisel, I do not. This was close enough for me 

This part is pretty critical. Laying out the mortise. There needs to be a 3/4" deep mortise along the entire breadboard, and the long tenons need to be 2" deep. The circled marks are where the actual tenons start. The lines which the arrows point to indicate the size of the mortise. I am leaving about 3/8" of room on either side of the tenon for the top to expand and contract inside those mortises. 

Then I layout the mortises. I have 3 lines here because I am going to drill out the waste. If you are using a mortise chisel, the center line is not needed. I like to have the center line so I can line up the point on the drill bit

Now I sent up a fence on the drill press and drill the deep mortises first. After that is done I reset my depth gauge to drill the shallower mortises 

Cleaning up with chisels. I like to use a mortise chisel to bang out the waste between the drilled holes, then pare the walls with wide chisels. It's important to support the board. These are deep mortises and the sides can split easily. I have some scraps clamped to the edges, and the piece supported in my birds mouth 

Then I clean out the mortises staying to the gauge line and checking the depth

Complete - now to pare the waste off the edges of the panel so the breadboard can mate flush to the baseline

If you look closely at the marked lines on the breadboard and the orientation of the tenon, you can see the amount of room for expansion and contraction. I then tap it home. The fit should be fairly snug where you need a mallet to get it on. If it seats with only hand pressure, it's a little loose.  

This video is the other side of this panel, but demonstrates the right amount of force needed to seat the tenon 

Good fit. This is what I meant by a blind tenon. Once I trim these to length, the joinery will not be visible. 

For part 2, I will trim the breadboard ends to length and then pin them in place.