A few instructions can save hours of head scratching - so here are a few instructions.
What are you waiting for!
Before laying out any dovetails, you need to prepare the ends of the two boards that are being joined. Because the sides slope, finding the correct angle to cut the ends takes a little geometry. What's more, it's not just one angle, but two! So take a deep breath, and jump right into the next step.
Grab yourself a large piece of paper, and draw an accurate elevation (front view), with an aligned plan (top view) a little way underneath.
First we'll find the width of the squared four sides stock you need to start with. On the drawing, extend the inside face of the side running in and out of the paper upwards. Now draw in a line that runs down, perpendicular to the extension, which connects to the top, outside edge. (The picture explains this much more clearly!) Now you can measure off the full width, as shown.
It may seem strange, but you can't simply measure the length of the board required from the top of the elevation, thanks again to the sides sloping. But don't worry, it's not too difficult. Draw in a line, parallel to the top, from the point that gave the full width, out past the end. Then extend the outside face of the side to meet it. Now you can measure from this new point to the opposite end of the elevation. REMEMBER to construct a similar point at the other end first, if that is angled too! Again, the picture explains this better than words.
The angle which you cut the face of the boards needs to be found now. There is a little more going on to find this, but again, the picture paints a thousand words. Rotate a copy of the side in the elevation (the one going in and out of the page) 90°ccw. The corners of this copy can then be projected down into the plan, intersecting with the top and bottom edges of the inside face of the rear side (extended towards the left). These four points of intersection define a true, flat, view of the end of the side, from which the cut angle can be directly measured, or transferred to a bevel gauge.
One last angle to find before we can prepare the sides. In the previous step, we constructed a true, flat, view of the end of the side. By intersecting this at right angles, with two parallel lines that are the true thickness of the boards, the diagonal between the intersections gives the bevel angle of the joint edge. Without this bevel, the joint would have a gap on the inside when the parts are aligned at 90°, due the way the angled end issloped. Try it and see for yourself.
Cut each side to length, with the true end angle(s). This is easy with a back saw, or miter saw. Prepare an angled auxiliary fence (90°minus true end angle) for a shooting board to shoot these accurately.
To bevel the ends, either plane freehand using a bevel gauge to check, or make a ramp (90° minus the bevel angle) to add to the shooting board setup.
You can't use a normal dovetail template, because the angled ends throw the alignment out - the resulting tails would have one steep and one shallow side, leading to an odd looking and weak joint. Instead, each side of the tails are laid out at the same angle to lines drawn parallel to the side's length.
Before spacing out the pins and tails, use the bevel gauge, set at the slope angle, to mark the bevels that will be made later to level both top and bottom of the sides.
Then mark an allowance for this on the face of the sides. This will avoid creating weak outside pins.
Now use the bevel gauge (set at the true end angle) to scribe the depth lines, for the tails on the tail board, (and pins on the pin board).Decide on the pin thickness, and mark one inside the allowance lines just made.
Now divide the remaining distance by the number of tails required. I do this by angling my rule such that an easily divisible number fits the space, marking the divisions, and extending them parallel to the side's length.
Set in the 'waist' of each tail - I'm using a 1:6 slope here - and join up with the outside corners to define the tails. You can do this for each tail, or just the first one and then use a bevel gauge to copy to the others.
With the bevel gauge set to the slope angle again, you can transfer the tail layout across the end. However, because of the beveled end, the stock of the bevel gauge should be held flat against the face, and the blade left to float (see photo).
This is because with the blade flat on the end, the slope angle isn't the same, as you can see in the second photo, where the edge has been planed to the correct angle for it to sit level.
Once the layout of the tails is complete, take the time to mark in the waste - sawing the wrong side of the line is easily done if this precaution is not taken.
I use a jeweler's saw or coping saw to remove most of the waste, before cleaning up to the depth line with a chisel.
Make sure a straight edge can sit on the depth lines on both faces at the same time, to ensure there is no 'bump' to foul the joint going together.
Lay the tail board on the end of the pin board, such that both are aligned correctly, and transfer the tail profiles to the end.
These markings can then be extended parallel down both faces to the depth line. Once again, mark in the waste. A knife is best for accuracy, but following up with a sharp pencil may help you find the knife line more easily.
If your sawing and transfer marking were good, and the waste removed fully to the depth lines, then the fit should be assured.
Plane the top and bottom bevels, marked out earlier, such that the assembled joint will sit flat and level.
Once cured, the excess glue can be removed, and the sides flushed with the tails and pins, either with a plane or sandpaper.
Now watch the video.