Well, I'm assuming you just need a satchel type... Patterns and measurements are lovely and all, but the best advice I can give you is to sew the material inside out, so the seams get left on the inside.
The flat type (heaps simpler) simply needs you to lay two pieces of material together and sew up the sides of it, attach a shoulder strap and to just invert the whole thing.
The type with a flat bottom, sort of like a box, is a little bit trickier. In essence, cut out your base with the right size first, and measure the perimeter of this piece.
And when you cut the fabric to size, ALWAYS leave a centimeter spare fabric on each side for seam allowance!
If you want the sides of the bag from one piece, simply make the side panel this long. If you want it from two (or more), divide the measurement by the number of panels you want. Pin the sides flat around the outside of your base, and sew.
Sorry if this isn't what you wanted! Too many people I know flee in terror at the word 'sewing', so I thought a basic lesson might be in order. Let me know if I was too generic about anything! aND BEST OF LUCK.