Be sure to start out doing each step slowly, carefully, and CORRECTLY. This is the only way to ensure that your muscles and nerves learn the correct movements properly. Unlearning the wrong movements takes much longer than making sure you have them right in the first place. Speed and stopping power will come as you gain confidence and familiarity.
Mentally think of Step 1: ready position, Step 2: scissor, Step 3: engage brake, and Step 4: sit down each time you go through the stopping process. After you've done a lot of stops over a few weeks you should find that the whole process comes much more naturally.
Practice the stop every time you go out skating.
Ready position:
Once you have the stop down pat, try the following more advanced steps:
Smooth out the transitions between each step, and do them a little more quickly. Don't skip any though, else it's likely to go wrong.
Scissor:
Increasing the braking force by sitting down more quickly on step 4, and eventually by shoving your front foot down and forwards into the ground. Remember it's much more effective and easier to use your weight to push the brake into the ground rather than your calf and thigh muscles.
Slide:
Together with this, make a chalk mark on your practice surface and try to stop before that mark. You'll soon find yourself able to skate faster and faster, and still be able to stop before the mark.
Sit down:
Try out stopping on some rougher surfaces, such as rough tarmac, paving slabs, tiles, etc. Be careful about your brake juddering or catching, but you'll be amazed at how easy it is to stop on these non-ideal surfaces.
See if you can make that brake pad screech and even smoke!
