When looking at a large number think of it as blocks of three, starting on the right. Each block of three is hundreds, tens and units.
The first block is hundreds, tens and units (of ones or units).
The second block is hundreds, tens and units of thousands.
For example: 456321.
The 321 is easy: three hundred and twenty one.
The 456 is easy, four hundred and fifty six – but it is 456 thousand.
So 456321 is four hundred and fifty six thousand, three hundred and twenty one.