Use the same style and format as your resume.
If you look at the sample image, you'll see there's a header at the top of my example sample sheet. This header directly mimics the format of my resume, and throughout the sample sheet I use the same fonts and styles as in my resume for the sake of design consistency.
Select images that can still be viewed cleanly at a smaller size.
Unlike in your portfolio, you'll be shrinking these images down to fit numerous pieces onto a single letter-size piece of paper. Don't pick artwork and video captures that require a high-resolution view of detail to truly appreciate. Choose images that scale well.
Showcase no more than 10-12 images.
This isn't meant to show your entire portfolio or demo reel, but instead meant to entice them to want to see more of the material in your portfolio and demo reel. It can be hard to pick and choose, so depending on the type of job you're applying for, you may want to have more than one sample sheet that highlights different skills. Make sure to use a good mix from both your print portfolio and demo reel stills.
Either show one image per project, or X number of images per project, but be consistent no matter what you do.
If you have a wide range of skills you want to showcase, pick one image per project and just show a wide variety of projects. My sample shows nine different projects, each with one thumbnail per project. If you want to show in-depth images from specific projects, though, decide how many thumbnails you want to show per project, and stick with that; don't bounce between one image for one project and three for another. You may want to have a row of four thumbnails from a single project, and nothing else on that row - so that each row of images discusses an individual project.
Make sure to include titles and brief descriptions for each project.
No more than one to two lines, depending on how much space you have - but it helps to place them in context, especially if it's paid work instead of just images you did on your own time. IF there's space, you can discuss methodologies and techniques.
Use tables to organize your sample sheet.
You'll likely be putting this together in Word. Use tables to make sure everything is evenly spaced both vertically and horizontally. Personally, I'd recommend tables without borders, so everything looks naturally placed within the white space. Borders tend to make things confined, and can highlight the size discrepancies between images with different proportions.
Never go over one page.
Remember, it's a sample sheet, not sample sheets. Stick to one page and one page only. Any more is just going overboard; it's like having so much information on your business card that you need to hand someone two business cards with your contact details. It just doesn't work.
When sending a print resume, always print in color.
Unless your entire portfolio of work is in black and white, you'll want to showcase that sample sheet to the best advantage, so make sure to use color prints. If you have to go to a copy shop to do that, do it. It's worth the extra few cents.
When sending a digital resume, include the sample sheet as part of the resume file.
Don't put the sample sheet in a separate file; they may or may not open it. If it's the last page of your resume, though, it's right there for them to review as they're looking at your experience.
That should be it. Make sure to do a few test prints to make sure it comes out decently, and good luck.