You know the scene: You give your 18-month-old a warning, he smirks and jerks the dog's tail again. At 18 months, your child is just starting to understand the relationship between his behavior and consequences. How frequently you need to discipline in the future depends on how effectively you discipline now.
Explain your tot's infraction in as few words as possible. For example, if he just launched a block at his sibling's face, kneel down to his eye level and firmly say, "No. We do not throw things." Resist the urge to scream at your little one. He's less likely to understand or absorb a message that's delivered in white-hot anger.
Carry your tot to the designated time-out spot immediately after explaining his offense. The longer you wait, the less effective your discipline will be. Choose a time-out spot that's safe but away from the scene of the crime and any older siblings who might taunt or tease. For a child who's just turned 18-months, a designated chair or step works well.
Place your child on the time-out spot calmly but firmly and explain that he is in time-out for two minutes for doing whatever he did. Some 18-month-old children will remain in time-out calmly and quietly for the entire two minutes at which point you'll exchange hugs and he'll apologize to the injured party. However, the more likely scenario is he'll either bolt from the time-out spot like a rabid squirrel or thrash and scream so loudly that you're tempted to respond in a way that would guarantee prison time.
Return him to the time-spot, once, twice, or thirty-five times if necessary without speaking. Be firm and persistent when returning him to time-out. Smirking or scooping him up playfully at any point in the process tells your child that this is a fun game and that if he keeps playing you'll stop returning him to the time-out spot. If he flails and screams in a way that's likely to cause injury, sit in the time-out spot yourself and hold him in a seated position on your lap. The time-out ends when he's calm for at least 15 seconds.