Step 1: Pick the big picture
Big picture is the picture your photo mosaic will look like. Keep it simple. Rule of thumb is that it will be a good "big picture" if you can make out the picture as a small thumbnail. Crop your big picture into one of many common frame sizes. Your photo mosaic will have the same aspect ratio as your big picture.
Step 2: Pick the small pictures
Small pictures are pictures that make up your photo mosaic. Your tile photos are square, so if the default center crop doesn't look good (i.e. cuts someone's head off), then move the crop area, and crop it. You don't need to crop it if the default center crop looks good. Use as many photos as possible with various different colors that match the colors of the big picture. There's no limit on how many photos you can use. More the better.
Step 3: Make photo mosaic
Make Photo Mosaic
This part is easy. Just click the "Make Photo Mosaic" button, and Mosaically software will start analyzing all the colors of your photos and start arranging them to make your photo mosaic. It will take a few seconds to a few minutes depending on how many photos you're using. Photo mosaics make truly amazing personalized photo gifts.