How to convert random bytes into blocks of numbers without weakening entropy? Let's say you have a, or several random byte(s) 0-255 and would like to derive numbers 0-9 for a One Time Pad (OTP) of them.
What would be a non-naïve approach to represent 256 possible values onto 10 possible values without weakening the underlying security? I am not sure that I simply can discard 6 states of the 256 values to achieve division by 10 without a fraction as result and not weaken security at the same time.
Is there a simple, yet convincing mathematical approach that I could port myself to a programming language to map bytes into blocks of numbers ranging from 0-9 safely?
Yes, it is called the simple discard method or rejection sampling. Basically you generate a byte of randomness, then check if the value is equal to or higher than 250 (the highest multiple of 10). If so, you discard the value and regenerate the byte. If you have a value in the range [0, 250) then you simply use modulo 10 (the remainder after the division). And there you are, a well distributed and secure value.
Of course, this method is not very efficient when it comes to a one-time-pad. Generally the one-time-pad is used over bits. But if you have it over digits then it is easier to use 10 to the power of digits you need, then use the simple discard method to generate the larger value, and then perform the (optional) modular reduction. If you take just enough bits, the modular reduction is not needed. After generating the binary value you can perform base conversion to convert the binary value into digits.
If you think the second method is still not efficient enough, then you can use the optimized simple discard method of which I'm the author.

Note that many libraries have both a method to create a large random number in a range as well as methods to get a decimal string, so in that case you can retrieve all required random digits in just two calls (although you may have to convert the character string into values within an integer representing each digit).