I used to have the HI 713, but upgraded to the 736. I like the increased precision, and with an sps tank this is a must. You will see a lot of varying opinions on Hanna, bottom line is, if you're very precise with the test, then you will get accurate results. I've always read to use the same glass vial for step one and two--minute differences between the vials can skew your reading. The old meters timed out after 2 minutes, which plenty of time, but just know this ahead of time so you don't waste reagents. The new meters time out in 3 minutes so you have even more time.
Basically, you put in the vial, press the button. As soon as it says add C2, start a timer for 3 minutes (so you know how long you have till time out), have the reagent packet pre-cut/open, pour it in and swirl it until all is dissolved. Usually only takes a minute or so. I would not advise to shake vigorously as this creates a ton of microbubbles, which can be a pain to get off the vial. Make sure no microbubbles are on the sides of the vial when you put it back in, or else you'll mess up results. I know it sounds like a lot of instructions, but it's really not that bad.