First off, the speed will come. Especially on your first day "on your own," you will probably feel overwhelmed. That's ok. It will pass. It might take a few months, but eventually it will all become second nature.
As far as some general tips:
-Try and finish the HPI, ROS, and PEx before leaving the room. This won't always happen, and will take some time. If you can finish the MDM, bonus points. But if you get at least the first three done, it's easy to come back and finish the MDM.
-For the HPI, remember to only include pertinent parts of the patient's story. Patients, unfortunately, don't dictate notes well. Their stories can wander here and there, and you probably don't really care about certain things. Tell a story here; this is your chance to paint picture of what's going on. Make sure it's complete, but some things just aren't relevant.
-For the PEx, the doctors you work with will probably either 1) note findings aloud in the room or 2) tell you when you leave. Particularly for the first option, don't always write things down exactly as they say them. For example, they might tell the patient, "You've got a pretty good scrape on the back of your head." Don't write that down. Write down "1x2 cm [get the size outside the room if they don't say it] abrasion to occiput. No active bleeding noted. No crepitus or deformity [etc..]."
-For the MDM, this is where you list your differential diagnoses, why you (well, the doctor, but you're writing it, depending on where you work) feel that XYZ test would or would not benefit the patient, etc. Defend the course of care with the court and other doctors in mind. Document important discussions with the patient (refusals of care, imaging risks, etc.).
-If it's going to be a quick discharge (e.g. you are working in fast track), try and get the discharge instructions completed before leaving the room before working on the MDM, so the doctor can just take over the chart and print them out quickly if they are moving fast.
Above all, don't be afraid to ask questions. Obviously, make sure your timing is good (don't ask right as he/she is about to do something or is thinking through an issue). But generally they are happy to explain things or give you more information. Good luck!