Child-proofing the environment is not a set rule. Accidents happen, injuries occur, all dangers cannot be avoided. You can, however, remove the most obvious and potentially destructive objects and opportunities. At the same time, your toddler needs to learn to stay out of things, not to pick up everything lying around, to refrain from tugging at or climbing on electric cords, and the like. How do you manage this learning? Even though it is not possible to give your toddler your complete attention all the time, check on him every few minutes just to see what he is doing and to make sure everything is alright. Become aware of those times when things are too quiet or too noisy. If your toddler is being especially quiet, he is probably asleep or getting into something he should stay out of. If there is excessive noise and banging around, there is also a strong likelihood he is into things he should learn to stay out of.
As you focus on the expanding circle of your child's world and the boundaries of that circle and the space within it, you see it is quite acceptable for a toddler to play (with supervision) in the water in the bathtub, but unacceptable for him to play in the water in the toilet. It is alright to play and experiment with toys but not with objects on your desk or kitchen sink. It is acceptable for him to get things out of the drawers or off the shelves in his room. It is not acceptable for him to get knives or other cooking utensils out of the kitchen drawers or to play with the cans of food on the kitchen shelf. It is alright to go through the doorway from the living room to the kitchen but it is not acceptable to go through the frontdoor to the street. It is acceptable for your toddler to hit his doll with a straw or newspaper but unacceptable to hit the cat with a stick. (If the rule at your home is 'We don't hit,' just be sure it applies to everyone, all the time, including hitting dolls and anything else. Hitting people is never okay.) The world of limitations and expectations is indeed confusing.
As an infant, your child is allowed to do most anything he wants. For your toddler, this world of nearly complete freedom exists no more. For him, the world has suddenly become divided into acceptable and unacceptable behavior. If this weren't enough, the rule makers (parents) aren't totally consistent about these limits. Sometimes it is alright to crawl into the other room and sometimes it is not. Sometimes it seems acceptable to play with one's toys and sometimes not. If the boundaries and limits are confusing to your toddler, how can you help him learn about such things?
A basic step is to organize your toddler's environment so as to minimize danger. If she is allowed to play with the toys on the shelves in her bedroom, be sure heavy toys are on the bottom shelf, the shelves are stable enough to permit climbing, and objects in which she is most interested are easily accessible. Be sure openings to stairways are closed to her. Keep all poisons and other dangerous substances on high shelves, preferably locked. Do not leave a toddler in a room where a heated iron is plugged in or where she might get into a heater or other dangerous apparatus. It is good judgment to place especially prized objects out of her reach. Look around toddler's immediate environment to see what might be gotten into, climbed on, pulled down, thrown, put in the mouth, or otherwise used by toddler in a dangerous way.
What happens once you become aware a toddler is about to do something he shouldn't? Start by firmly telling him no. Next, gently restrain him from continuing the undesirable behavior. Then, watch him for a minute or two. He probably goes back to doing it. As soon as he starts back, repeat the process several times if necessary. As a last resort, pick him up and take him into another room, put him in his playpen. He learns this particular behavior or playing with this particular object leads to negative consequences.
As your child gets older, the number of boundaries and limits declines. Nine- and ten-year-olds can handle most environmental situations and do not have to have most dangers removed. Nonetheless, the boundaries or limits need to be enforced. Taking your child by the arm and removing him from a dangerous situation becomes, for your adolescent, not allowing him to drive the car for a week or so; putting your child in his playpen becomes grounding your adolescent for a few days; and gently restraining your child shifts to the withdrawing of privileges (social or athletic functions, for example) or the prohibition of certain activities. Learning about boundaries and limits starts in infancy and continues through adolescence into adulthood.
Any time you consider the imposition of a boundary (especially with older children) consider first whether any limit is reasonable and appropriate. Doing nothing may be appropriate. If the limit is appropriate, then follow through in a reasonable way. Reasonableness and appropriateness are related to your child's age and to the particular behavior. What you can do expands as your child gets older. For example, by the time he is five or six, it may be enough to talk with him about the danger or the problem.
Although you set and enforce firm limits and prohibitions along with other expectations you have for your child, there are a few things you must NEVER do no matter what your child does or does not do. With added emphasis, this includes never permitting anyone else to do any of these things to your child.