The Original Stop
Our original stop for a buy is just below the previous day's low by 1/8-3/16 and vice versa for a short. If this price is just off a whole number we will place our stop 1/8 on the far side of the price. For example, let's say we are looking to buy a stock at 40 1/8 and the previous day's low is 38 1/8 we would place our stop at 37 7/8 because the whole number 38 will serve as support. We place it 1/8 below that whole number because it can often break it by 1/16 by people that get too antsy, but it typically breaking it by 1/8 will signify that the break it real.
The other original stop used is most commonly employed in the event of a gap past your original entry point where you use a 30 minute rule to enter a stock. In cases such as a buy, where the stock gapped up 1/4 above our buy price and we entered it when it broke the 30 minute high, we will most often use 1/8-1/4 below the established day's low at the time the stock sets up as the stop.
Make sure you adjust the 1/8-3/16 amount according to the price and volatility of the stock. To a stock over $200/share, 1/8 is nothing so you want to place your stop lower than 1/8-3/16 under the previous day's low (or current day's low.) In this case $1 would be more reasonable. Similarly, in more volatile stocks you also want to place your stops a bit further away.