Before you get started, make sure you know what size index cards you want to create in Word, i.e. 3" x 5", 4" x 6", or some other size. You can even choose a custom configuration and print the cards out on heavy cardstock if you wish.
Step 1: Go to the Page Layout tab on Word's ribbon. Click on Size in the Page Setup group. (Click any image in this tutorial for a larger view.)

click to enlarge
Step 2: Scroll through the list of available preformatted sizes and see if the layout you need already exists. If it does, select it and go on to
Step 4. If not, choose
More Paper Sizes from the bottom of the list and continue with
Step 3.

click to enlarge
Step 3: If you chose
More Paper Sizes in
Step 2, the
Page Setup dialog box will appear on your screen. Make sure that the
Paper tab is selected and enter new values for
Width and
Height as shown in the screenshot below.

click to enlarge
Click
OK when done.
Step 4: At this point, you'll be returned to the Word document with your new page dimensions applied. If you want to change the orientation of the layout, you can do so on the Page Layout tab of the Word ribbon. Just click Orientation in the Page Setup grouping and choose between Portrait or Landscape.

click to enlarge
Step 5: Begin typing the information you want to appear on the index card. Here, you can change the font,
create a table, or do anything else that you normally would be able to do in any Word document.