Know your flash's range. This is a very important technical aspect, that is mastered by experience.
Making a digital photo collage is not as difficult as one might think. It's really very simple with most photo editing software. It just takes a little time and patience. Even basic photo editing software that normally come with the purchase of a digital camera or printer. A very good photo editing software is PhotoShop Elements and older versions of it, which are perfectly fine, often come included with a new digital camera, printer or scanner.
Photoshop Elements is the software I have used for my step by step guide, How to Make a Photo Collage.
The basic steps in making a collage are simple. The first step would be to open a blank document in your photo editing software. Your blank document should be the size you want your finished photo collage to be, example, 8x10 and of a resolution of 300 for good printing quality.
Next choose your pictures, keep it to about 10 pictures or less to begin with, till you are used to working with a lot of pictures so as not to be overwhelmed. Open them and drag them into your blank document.
Next resize the pictures as necessary. A tip, hold down the shift key as you drag from a corner to resize the picture, holding the shift key will keep the proportions of the picture so as not to distort the image. You can resize back bigger or smaller as needed as you work to decide the layout o the photo collage. Move the pictures around to see what looks letter where, try to create some balance, perhaps choose a focal picture to place in the middle and be larger than the others. But the focus does not have to always be in the middle, see what works best with the pictures you are using.