I've made my own enlarger, and while I didn't do a very good job, it was more a woodworking problem and a photography problem. An enlarger is kind of the inverse of the camera. Light shines through the film, through a lens onto paper. Here's a few basics to keep in mind. You'll need a box to hold everything, and keep the light from hitting your negative, except through the enlarger lens. You can use a typical room bulb as your light source. You'll need a diffusion material to spread out the light and make it even. I've used a piece of opal glass, but a piece of plate glass sanded until it is frosted, or even waxed paper is ok (as long as you keep it far from the bulb so it doesn't burn). Then you need a stage for the negative. I've taped it before to a board with a hole in it the same size as the negative. You'll need a lens and a focusing system. I suppose in a pinch, you could open the back door of your camera, find a way to hold the negative in place behind the lens, and use the bulb mode on the camera to hold the shutter open, using the lens to focus. It's also easier to make an enlarger which projects on the wall. That way you don't have to worry about holding it up, having a way to raise and lower it to change the negative size, and keeping it stable. Instead just move it further or closer to the wall, change the focus on the lens accordingly, or look up "sliding box camera" to get an idea of a simple focusing mechanism for lenses. I think the key point is to simplify as much as possible.
Most of the enlarger's complexity comes from versatility. A comparison between the box camera and the SLR is apt here. The box camera has one shutter speed, one aperture, and a fixed focus distance. If you keep within the parameters of what the camera is capable of, you can make some fine images. So instead of trying to build the multi-speed, multi-focus SLR, start with the enlarger equivalent of a box camera. With skill and discipline your results can be quite good.