Let N is block size. For example its equal to 32. So we need split text into 32 bits blocks and represent each block as positive integer from 0 to UInt32.MaxValue - 1. We can use UInt32 to store 32 bit block values.

If you want to use different value for N you should think about data type for store block values.

You should make sure plain text length is multiple of N.

Remember: larger block means greater security