Every amateur astronomer at one time or another gets the bug to build a telescope. If you're looking to build an affordable high-powered telescope, the simplest and most cost effective is the reflector telescope. Build the biggest bang for the buck scope you can.
Figure the focal length of the primary mirror you are using. If, for example, you use a 40-inch focal length mirror, the measure from the surface of the mirror to the spider then up through the focuser to the eyepiece will need to be 40 inches. Set the focuser on top of the sonotube. Measure from the top of the focuser to the tube. Add half the diameter of the sonotube and subtract the total from the focal length. This is the first measurement you need. Add this number plus 4 1/2 inches plus the depth of the primary mirror plus the mounting cell.
Cut the sonotube to the length you just figured. Spray the inside with black matte paint to reduce reflectivity.
Test fit the mirror-mounting cell in one end of the sonotube. Square the cell with the end of the tube and mark the holes for the mounting screws. Mount the mirror onto the cell mount as specified in the instructions that came with the mount. Slip the mirror and cell into the end of the tube and screw it into place, centering the cell in the sonotube opening.
Mount the secondary mirror on the spider mount following the instructions. Measure from the center of the secondary mirror to the legs of the mounting spider. Subtract this distance from 4 inches. This is how far from the top end of the sonotube to drill screw holes for the spider legs. Drill four holes in the sonotube that far from the top edge of the sonotube and equidistant from each other so the spider legs mount perpendicular to each other. When drilling make sure the open end is below the mirror and cell so dust and debris don't fall on the mirror. When screwing the spider in place, adjust the mounting screws so the spider mount is centered in the main tube.
Drill a hole in the sonotube for mounting the focuser with a hole saw. The hole must be directly over the secondary mirror and the same diameter as the opening in the focuser. The center of the focuser hole will be the distance from the center of the secondary mirror to the spider legs below the spider leg mounting holes.
Screw the focuser into place over the hole you just drilled. The secondary mirror should be in the center of the focuser tube when you look through it.
Collimate the telescope by adjusting first the primary mirror with the collimating screws on the bottom of the mirror mounting cell, so that looking down the sonotube from the top the mirror reflects the image directly back at you. All four spider legs will line up to give you a rough alignment. Now look down the focuser tube. Keep adjusting the collimating screws until the secondary mirror is centered in the focuser tube. If you've got it right, your eye will appear in the center of the secondary mirror.
Screw the finder scope mount to the tube 1/4 way round the tube from the focuser, so that it is lined up parallel with the tube, easy to reach and not obstructed by the mount or the eyepiece.
TL;DR (Too Long; Didn't Read)
You can obtain sonotubes from the lumberyard's concrete supplies section. Work slowly. Measure twice for every cut to make sure everything lines up. Brush dust off the optics with a fine camel's hair brush or blower. Clothes will scratch and dull lenses. Spend the bulk of your money on a big mirror, eyepiece lenses and big focuser that will take 1 ¼ inch or 2 inch eyepieces. Mount the cell and spider securely and center them. Good alignment is critical to a crisp image.