I agree, practice.. practice.. practice... It's the only way.
Be sure you have the correct size cuff, that you are looking eye level with the mercury, and that the mercury column is at zero before you begin to inflate the cuff. The suggestions about earpieces pointiing toward your face and being the rigth size are important too. The more times you physically take blood pressures helps you get a good feel for the actual pumping and stethoscope holding, etc. Once you are good at that part, you can begin to spend more time listening very carefully.
Once you become proficient doing manual blood pressures, you'll realize that you have the gift of being able to access incredibly important information especially during a potential crisis situation. The blood pressure is a quick and incredibly valuable tool to assess patients with. Changes in systolic and diastolic pressures as well as things like widening pulse pressure can be some of the most valuable information you can have to use your critical thinking skills to recognize a patient is headed for trouble before he/she is actually in deep trouble. It's great to know you want to learn this skill well... it shows you have good nursing intuition already (in my humble opinion).
As far as hearing the sounds... that's the most difficult part for most people. The only way to get past this is to practice on as many people as you can. Friends, family, other students, sunday school classmates, whoever will let you!
I'd suggest you ask your instructors if they know of any health fairs you could attend to help take blood pressures. In nursing school we held our own health fair on campus to help raise travel money to our NSNA convention. It was a great way to get to take blood pressures on healthy, young people. Your instructors may be able to find a way for you to participate in a hospital or other community based health fair too. Being students, an instructor or another person would be available at the fair to double-check any blood pressure result you got that you felt uncomfortable with... or had problems obtaining.
As Jacolaur mentioned: Never, ever guess!!!!! This is so important!!!!! NEVER, EVER GUESS.
Our nursing instructors had a stethoscope with one tubing and two sets of binaurals and earpieces on it. For our checkoffs on B/Ps we'd do a blood pressure on someone while our instructor listened and watched the mercury at the same time. That would be a great thing to use to practice with along with another student... especially if that other student was more proficient than you are. Maybe a student from a class that will graduate before you would be willing to help you...
Didn't mean to ramble on like this. I would like to say that it's great to see you wanting to learn to do manual B/Ps well, because it tells me your nursing intuition is beginning to kick in already... you are able to instinctively understand the importance of this nursing skill and the impact it can have on your career.
You can do this. Just keep practicing and tell yourself, "I can do this... after all, millions of other people have been able to learn this before me, and they are no different than I am!"
Best wishes for a rewarding nursing career!