Setting up the plane for it's final descent and approach speed is actually pretty simple. Many instructors give students various different speeds for the different legs in the pattern, with each speed being less as they get close to final. I don't teach this method because I feel that it gives the student more to think about.
Next time you are on downwind, look at the horizon and with your best estimate, guess how many inches the top of the instrument panel is below the horizon. Depending on how high you are sitting in the seat and what your power setting is, it may vary somewhere from 1" - 3". For this example we will assume that the top of the panel is 2" below the horizon in level flight on downwind.
With that number in mind, when it's time to throttle back, all you have to do is lower the nose about 1". So if you were flying with the instrument panel 2" below the horizon, you will now have it 3" below the horizon. If you hold that pitch angle accurately the whole time, even while turning, the plane will slow down to the correct airspeed all by itself as you add flaps. You have to adjust your throttle as needed for altitude. Typically for a small 4 seat plane, the power will be 1500 - 1600 rpm.
An example in a Warrior would be 2300rpm on downwind. Just before I get even with the numbers on the runway, I power back to 1600rpm, drop the nose 1" lower and put it a flap at the same time. Don't allow the plane to go up when you put in the flap! Just before I turn base, I put in the next flap. Keep it trimmed in that nose low attitude after each addition of flaps. Just before turning final, I put in the last flap. If your speed is increasing while descending instead of decreasing, then your nose it a little too low. If you're descending 1000' per minute, then your nose is also too low. Many people end up raising the nose during the turn so be careful not to let that happen.
If you hold this position all the way around, then you will get a gradual decrease in speed while adding flaps, and end up at your final approach speed by the time you turn final. The altitude should be close too but you need to keep an eye on it and make small adjustments as needed with your throttle to maintain a good glide path. This method works great. I'm not sure how it will transfer to you in written form without a demo, but if you're having problems controlling your speed and altitude when landing, it would be worth trying. Put the plane where it should be and everything will fall into place by itself. Have fun!