The concept of slowplaying a big hand is so commonly misused by beginners.
They have a big hand they think "great, I will slowplay and trap him."
A lot of the times it wins you less money and other times still it loses you the entire pot, which you would have won had you not slowplayed.
You want to slowplay, basically, when the following conditions are met:
1) You are unlikely to lose the pot.
2) Your opponent is unlikely to put a lot of money in the pot right now, given his hand range and the board.
3) Your opponent is likely to put in a lot of money, whilst still being beat by your hand, if you let him catch up a little.
That's basically it. Read those rules a lot of times to yourself. The bolded part is a part I seem to see a lot of beginners struggling with.