How to place stopper nuts
Nuts are a great passive type of protection. When placed correctly they are solid and many times can be better suited for crack protection than cams. Stoppers are super strong and have come a long way since they were invinted in the 1970s. A great deal of shapes and sizes area available along wtih offset sizes to augment regular stopper sizes. Nuts are great protetion and a necesity if you lead trad stopper nuts are practically a necessity.
Find solid crack with good quality and not loose or flaky rock around it.
Look for an hour-glass shape that is wider a the top and narrower at the bottom. Place a nut that is possible to bottleneck itself into the hourglass shape and seat itself securely.
Apply downward pressure to secure/test the placement. A good placement should be firmly wedged in the crack. Yo do not need to yard on or tug too hard to seat a nut. This is a misconception many climbers think. A good nut placement is seated with slight downward pressure.
Always be sure to place stopper nuts with a long enough draw or sling so it does not get pulled upward and dislodge itself out of the crack. You can also place an opposing nut underneath to prevent from pulling.
Many horizontals will accept nuts well. Look for a horizontal that has space behind it and a narrowing in the middle. You can also place a nut with a solid rock or pebble in a hirzontal to wedge the nut behind.
In any instance placing nuts you always want to practice with a knowledgable climber first then practice more until you are confident your placements will hold. You can find a climb where you can toprope first and practice placing nuts then have someone follow and critique your placements or even take small TR falls on your gear to test your placements and build confidence.