I've done this a couple of different ways. If you're doing it with B&W on a single sheet, it's not that difficult, it just takes some preparation. You will have to decide if you want the images to be centered on the paper, or whether you can live with them offset to the top edge. Also, whether you're going to matte the images with windows around each image, or show the image as a single piece with a matte around the whole edge will have an influence on how you handle the paper.

I prefer to show the whole two or three part image, centered on a single sheet - as the images then exist on the same plane uninterrupted by mattes separating the images.

I like to use a Speed Ezel to handle the paper. Decide how large you want the images on the sheet of paper and get the correct size easel for the paper. Cut a cardboard mask (like a matte) that fits the Speed Ezel opening with the correct image size cut out and centered in the easel opening. You know have created the image size you will be using. I like to use 8 ply board as it is thick enough to be handled, lies flat and won't slip under the Speed Ezel edges. I like the matte board as you can clean up the edges the on matte board using a piece of 220 grit sandpaper to get rid of uneveness or fuzz.

First, proof the images on separate sheets so you know the exposure, where you need to dodge/burn, etc. Make notes on this (of course) and refer to them when you do the final print - makes way less confusion as you only have to worry about the mechanics when you're making the final print.

One of the problems will be registration of the image. Either waste a blank sheet of paper or use a heavy weight smooth stock of some type - I just used a blank sheet of photo paper. So you can make a master layout.

On the back side of the paper, mark out how the images will line up (for spacing) and insert the paper (back side up) into the easel. Put your mask into place and line up the opening with the layout you've made on the paper. Mark a "1" in the center of the image area.

Now, pick some place on the easel and mark either the right or left top edge of paper as it hangs out one side of the easel with a small vertical straight line.

Move the paper to the next image position (center for a triptych). This is the tricky one because if it's truly centered, you won't have an edge sticking out of the easel. You will probably have some type of very slight offset that can be measured from the edge of the Speed Ezel to one edge of the paper. Or if you have enough of an offset you can make a second mark on the paper. I will make two straight lines (one under the other) to mark the second position on the paper - the two lines denoting the second image position.

Move the paper under your mask for the third position. Line up with your layout sheet and mark three straight lines for the third position.

Take your master out and measure where the lines are from the right or left edge.

Make a measurement "roster" on a sheet of paper. It would look something like this.

Image 1: 8 inches outside easel RIGHT
Image 2: 1/8 inch outside easel LEFT
Image 3 8 inches outside easel LEFT

Now, you focus the first image and center it as needed on the opening in the matte you've made. Take out your paper and clip off the top left corner slightly (or whatever corner makes sense to you) so you have the equivalent of a code notch or "key" so that you cannot accidently get the paper in upside down on a following exposure. Slide the paper into the easel and measure the amount needed hanging out as indicated on your measurement roster. Put your matte on top of the paper. Make the first exposure.

Take the paper out, put it in a light tight box or back into the package. Put in the next negative, focus align, etc. Put the paper back in to the easel and repeat for the next exposures.