Advice on teaching the two-digit numbers, including the handling of teen numbers.
Double ten frames recommended for showing students the numbers 11 to 19, to help them to deal with the inconsistencies in the number names.
The idea of "skip counting" in one place is useful for teaching place value, focusing attention on values in each place.
Use flashcards to help students to become familiar with two-digit numbers 11 to 99, including representations which show place value.
Understanding place value is a foundation for being able to work with multidigit numbers, especially in the written algorithms.