How to make any arbitrary matrix 'A' to Unitary matrix 'U' by Block Encoding method? Would be helpful if provided with an example.

Question

Block encoding is a method in which you are given a Non-unitary Matrix 'A'(it has complex entries and can be rectangular) and you make it Unitary by placing the 'A' matrix in the top left corner in a bigger matrix 'U' and filling the rest of the entries of 'U' such that the resulting matrix 'U' becomes Unitary. Then we say 'U' is a block encoding of 'A'. This image will would be helpful to picture it. Also there is a discussion over it in this YouTube video from 2:28 min to 5:54 min.