Counting a set of objects means applying one-to-one correspondence to a physical collection and arriving at the correct total. Children must organize the objects (e.g., move them aside or touch each one) to count reliably. The last number said when counting a set tells how many are in the whole set.
Provide collections of objects that can be moved (counters, blocks, coins). Encourage children to arrange objects in a line to avoid skipping or double-counting.
You already know two important things: the counting sequence (one, two, three, … all the way to twenty) and one-to-one correspondence (each object gets exactly one number word, and each number word is said exactly once). Counting a set of objects puts these two skills together to answer a practical question: how many are there?
The most important habit to build is organization. When objects are scattered on a table, it is easy to accidentally count one twice or skip one entirely. Moving each object to a separate pile as you count it, or touching it and sliding it to one side, ensures you apply one-to-one correspondence correctly. Arranging objects in a line before counting works well too — you can move down the line from one end to the other without losing your place. The physical act of separating counted from uncounted objects is not cheating; it is the right strategy.
The cardinality principle is the deep idea here: the last number you say when counting a set tells you the total number of objects in the set. If you count eight blocks and stop, there are eight blocks. This sounds obvious, but young learners sometimes treat counting as a performance (saying the number words in order while touching things) without understanding that the final word names the size of the whole collection. Testing this understanding: after counting eight blocks, ask "How many blocks are there?" A child who has the cardinality principle will say "eight" immediately. A child still developing it may recount.
An important discovery is that the order you count the objects does not change the answer. Whether you start from the left or the right, count the blue blocks first or the red ones first, you will always arrive at the same total as long as you count each object exactly once. This is called order irrelevance, and experiencing it with real objects — counting the same group in different orders and getting the same number every time — builds a deep, reliable number sense that will support all future arithmetic.