Before learning formal addition, children benefit from exploring what happens when two groups are combined. "I have 2 blocks and you have 3 blocks. When we put them together, how many do we have?" This is the foundation for understanding addition.
Use concrete objects (blocks, counters, fingers). Combine groups and count the total. Use both small and large numbers. Ask "How many now?" frequently.
Children may recount everything from 1 instead of counting on. They may not understand that combining two groups makes a larger group. Some may lose track of the original groups.
You know how to count objects in a group and find its total — the cardinality principle tells you that the last number you say when counting is how many there are. Now we take one step further: what happens when two groups come together? This is the very first idea behind addition, and it starts with physical objects you can touch and move.
Imagine you have a pile of 2 red blocks and a pile of 3 blue blocks. You know each pile's size by counting it. Now push the piles together into one big pile. How many blocks are there? Count them: 1, 2, 3, 4, 5. The key discovery is that this total is reliable — it doesn't matter which block you count first, or how you arrange them, the answer is always 5. Combining two groups produces a new, larger group, and its size is completely determined by the sizes of the original groups.
What you're learning is the part-whole relationship. The two original groups are the parts; the combined group is the whole. When you know both parts, you can always find the whole by combining and counting. Later, the addition symbol (2 + 3 = 5) will be a shorthand for writing this idea down using numerals instead of physical objects. But right now, the important thing is the experience of combining — pushing groups together, counting the result, and noticing it always works.
One useful shortcut to start practicing: you don't always have to count everything from 1. If you already know one group has 3 things, you can start at 3 and keep counting as you point to the second group: "3 … 4, 5." This is called counting on, and it saves time while using what you already know. It's fine to count from 1 for now, but try noticing when you can start from the bigger group — that habit will make addition faster once you start using numbers and symbols.