Hands are tough. It's easy to make them either too big, too small, or to make the fingers look like they're dislocated.
1. Map out the proportions using straight lines and angles. The three angles highlighted (in the first image) will really help position the hand accurately.
2. Block in lights and shadows.You do not need individual fingers at this point.
3. Modify. Add to lights. Add to shadows. Whatever level of detail you feel is right is put in here.
Step 1. Step 2. Step 3.