Get it right from the beginning, or a child lost forever. To begin with addition should be thought of as the combination of two sets to make a total set. Understanding vocabulary is important. Use the words, add, more, sum, total, altogether, equals.
The best way to practice addition is to do it practically, combining two lots of items to make a new total eg 3 sweets and 4 sweets joined together. At first children will probably count on one at a time until they reach a total.
Slowly they will realise that 3 and 4 always make 7 and this is something that they can learn and know.
After lots or practical work move on to paper exercises like the worksheets provided here.
The word equals is also important and should be explained as meaning “is the same as”.