Hint 1 - Figure out which was to press the HSTs before you trim, this will make putting them together easier. In examples 1, 2 and 3 all the seems should be pressed the same direction. In example 4 two of the HSTs are pressed in the opposite direction.
Hint 2 - It's very important to make sure that the diagonal seems interlock. This seems to be where mistakes begin, which throws off the rest of your block construction. The pic below shows exactly how they should meet. Then start sewing them together where the diagonal seems meet, that way you are assured that they will be perfect points.
Hint 3 - Once you have 2 HSTs sewn together finger press the seems open (no need to press unless you want to). If you pressed the seems in the right direction on you HSTs then this part will be easy. There's 3 places where the seems will meet. Both diagonals and the center. When I first started making these I did a test to make sure that the points in the center would all meet. I would just put a dot or small line in water soluble ink where the quarter inch seems allowance is. Then I put a pin thru the 2 dots to make sure they met. If one seem was slightly bigger than the other I could adjust accordingly. With practice you will find very quickly that you won't have a need for this step even, but its a good check when learning to piece these.
I hope these hints help you next time you are making HSTs. I'd love to hear your hints too.