1. Make a sketch design, something that you imagine...don't have to be detail...it's the basic idea that you have...This is my basic idea
2. Then, start to make the detail: pockets, may be? embroidery ribbons? colors? mix and match style?
3. Make the technical design: means that you must decide the size, making the pattern and start to figure out how to sew it.
My advice (i don't do it...but if you do, it will be better...) is to use a scale when doing this 3rd step...it can help you to figure out, in scaled size, how your bag will be looked like...
What i always do is: i don't use scale...but i always pull my measuring tape...see how long is 25 cm, 30 cm, etc, etc...then "sketch" the technical design, note the size...but not just an ugly sketch...this sketch must be readable because based of this, i will make the pattern...
i don't plan to sell my pattern, so, i don't really make a record that other people are able to understand...but if you want to make one that you want to sell, scaling is important and make sure your draw (design and technical) is understandable...
4. On the pattern, whether on the scale or real one, don't forget to note like:
* make it 2 for main fabric and 2 for linings, etc (depends on your own design)
5. Organize ALL: clip it, label it with design number or else...one day you want to make it again, just reuse the pattern you have...
Designing doesn't stop on the "draw" but it continue until you make the pattern...at least, even if you just draw it, make sure you can make it one day...it's your challenge: to make your "DREAM" comes true
(^.^)