Hmm, Now that you ask I am not sure. Diagramming is what I require of all my students. It is what is done by most professionals and I carry it a step farther since we are dealing with tiny optics.
Take a piece of drawing paper or other material (butcher paper, wrapping paper, inside of gift wrap etc.) Near one end draw in the mirror to size or scale. Preferably to size.
From the front center of the mirror draw a line perpendicular to the mirror out to focus. At focus, draw a line equal to your desired image plane which will be centered over the first line and parallel to the mirror. This line may be the size of the field stop in your largest eyepiece or as wide as your intended focuser. You may want to use 1 1/4" or 2" if you intend to fully illuminate that size of image.
I do not recommend limiting yourself to just the widest field stop of your current eyepieces unless you are sure you will never do better.
Draw in lines from each side of the mirror to the respective side of the image plane. This represents your cone of light.
Where the image plane is, you need to continue with a different (fainter)line extending each direction past the diameter of the mirror and a couple of inches more. Mark in the diameter of the mirror then add 1/2 the size of your image plane to each side. This is how wide the inside of your tube must be to avoid tube vignetting. You should also allow another 1/2" to 1" per side to allow for tube drafting and baffling. This give you an inside diameter and tube wall thickness will give you an O.D..
Some like to figure focus at 1/2 the rack out on the focuser. Measure that amount from tube to focus and mark that distance on the center line from the image plane to the mirror. Add on 1/2 the Tube O.D. and mark that on the center line. Now draw a line representing your diagonal at a 45° angle through the last mark. This gives you the placement and size of your secondary.
If you have drawn this to size, you can actually lay components on the paper and take measurements. This allows you to draw in your tube, mirror, mirror cell, diagonal,diagonal holder, focuser and drill points. This is the routine that I developed for my students and it never fails. Plus the components can be weighed and CG can be determined so mounting points can be placed ahead of time.
AKA whirlpool51 asked this nearly a year and a half ago. He might want to at least make a scope before volunteering advice in which he is still seeking...;^)