I've never found the built-in templates or the ones you can download from the guys who sell stickies and forms very "friendly" to work with, and generally just use a table in Word for smaller stuff or make a "picture" in an image editing program. I use Photoshop Elements for the image stuff but most any image editing program should do what you need unless you want to do some fancy stuff with the image on them. If you want fancy text, Word is about as versatile as you're likely to need, and you can type the text, print it, and scan it to get an image that you can crop and paste into a larger picture with the "graphic" stuff in it.

For the label to stick in a jewel box all that really matters much is that you get the right size square piece of an appropriate paper, and any method you've got handy can be used to print whatever artwork suits you on the paper.

I've generally found that I get a cleaner result if I print the pictures "to the edge" with a border, on a piece of oversized paper, and cut the finished square to size after the ink dries. A good metal straight edge and a box knife on one of the "self healing" pads from the sewing craft shops or from office supply places works well, although I generally use a "carpet knife" (i.e. a box knife with a BIG handle).

If you print the finished box dimensions as part of the picture you can either make a border (cut to the outside of the printed border box) or an outline that you cut off (cut to the inside of the box you print).

Most image editing programs allow you to "resize the canvas" and/or resize the image. You have a background color (often white) and a foreground color (frequently black). If you "flip" the colors so that what was background is foreground, and what was foreground is now background, and resize the canvas by increasing the size by the same amount each way, you get a very nice crisp border in the background color you had selected when you resized. No problem with "making marks" or "drawing lines" or other fancy stuff. Increasing width and height each by 0.04 gives you an 0.02 wide line all the way around.

Fancy borders can be made by "resizing canvas" to make a dark box around the picture, flip and resize canvas to make a light band, flip again to make a different width dark border, etc.; and you don't have to use just black and white. Trim at whichever "edge" you made the right size for your jewel box.

Note that I have found that jewel boxes do vary in the exact paper dimensions that fit best, and best fit also can depend on the weight of paper, so make a trial print first before you print the first hundred inserts, and make sure you've got it right for your boxes with the paper you intend to use. For some boxes a label that's "not quite square" may work better too (usually a small bit wider than tall when it matters).

You can also make "booklets" by printing "2 wide x 1 tall," stacking, stapling, and then folding. I find that a little extra margin, so you can stack, staple, fold and then trim the folded stack to length works better than trying to "print them to fit" since more than one layer means the paper in all the layers are not all the same length if you want even edges.

(Note that you'll probably need a "long reach" stapler for "booklets" with more than 4 pages.)

All you're really trying to do is make a (nearly) square piece of paper that looks like what you want and that doesn't fall out of the box too easily.

John