Can you make a photo? It is hard to imagine your words in my head.
I wish I could. At this time I have a foot injury that is keeping me from moving around and my shop is a hundred feet from the house.
Let me try to explain a different way.
If you angle your table saw blade to 45 degrees and then cut both ends of each side for the box that you will be building, so then each of your box side pieces will have angled cuts on each end, making the outside (top) of each piece longer than the inside (bottom). like this
\ _________ / .
Now you need to make your spline cut in each of the ends. If your saw is a right tilt the blade is already angled 45 degrees to the right. Lay the work piece flat on the table with it's shorter side down (like in the diagram above) and move your fence so that it is almost above the blade with the blade angling out from under it to the right. Now clamp a short block of wood to the right side of your fence to act as a stop for your work. It needs to be short enough not to reach the blade but can be tall enough to allow it to be clamped to the fence with the clamp high enough for your work piece to pass under the clamp.
Now place your first box side on the saw table on the right side of the fence, laying flat (as shown in the diagram above), with it's sharp edge touching the block of wood that's clamped to the fence.
Move the fence to the left until the saw blade is aligned to cut your spline slot in the correct place on the end of your box side, and set the blade height to cut the spline depth desired.
Now, using your miter gauge to feed your work piece, place it in the right miter slot, and clamp your box side (still oriented as above) to the face of your miter gauge, with it's sharp left end still against the stop block on the fence, and push the miter gauge forward to make your spline cut.
If you have done this correctly, you will have a spline cut in the mitered end of your box side that is at right angle to the mitered end and is the depth desired for your spline.
Now you can repeat the process to each end of each of your box sides.
For splined box corners, you really need splines with the grain running across their narrow width, not with the grain running length wise. This is very important to achieve strength in the joint, but it can be difficult to make splines like this of the correct thickness and grain orientation. Splines with the grain running lengthwise will not add strength to your corner joint.
For making cross grained splines the exact thickness needed for the spline cuts, I found that using my table saw tenon jig let me get cross grained splines of the exact thickness needed very easily. I set the jig up much like I would to cut a tenon, but it's actually the thin waste piece that I use for the spline. I clamp a donor board standing up on end in the tenon jig and set the jig so the saw blade will leave the desired spline thickness off the left side of the board with the blade raised high enough that the resulting thin spline will be 2X the depth of the spline cut that was made in each end of the work pieces.
After the first cut, the donor board can then be flipped over and the cut repeated to make another spline. Then the donor board can be flipped end for end and two more splines can be cut on the other end of the donor board.
To cut the splines free of the donor board ends, I set my miter saw to 90 degrees and place a stop so that the desired spline width will be cut from the donor board. Then I can make the cut on each end of the donor board to yield 4 cross grained splines for my project.
I usually repeat the whole spline cutting process as many times as needed to make more than enough splines because they don't always end up being the exact thickness needed, and they will break easily because of the cross grain orientation.
If your spline isn't long enough, there is nothing wrong with using more than one spline end to end to get splines running the full length of your spline cuts. I do this frequently, ending up with spline pieces sticking out both ends of the spline joint during glue-up. They trim/break off easily after the glue is dry.
Sorry, I don't have any pictures of the "in process" spline work, but the attached photos do show cross grained spline joints in 6 "cat urn" boxes that I built out of mahogany several years ago using the methods described above.