How to forge a textbook RSA signature? Let $pk=(N,e)$ (resp. $sk=(N,d)$) denote the public (resp. private) key in a plain RSA signature scheme. Define the function $\textbf{Int}: \{0,1\}^* \to \mathbb{Z}_N^*$ as follows: on input string $x=(x_1\parallel \dots \parallel x_n)\in\{0,1\}^t$ we set 
$$\textbf{Int}(x_1\parallel \dots \parallel x_n)=\sum_{i=1}^{n}2^{n-i}x_i$$
We also let $\mu$ denote an ASCII character to byte mapping in which $\mu(0)=0^8$, $\mu(1)=0^71$, $\mu(2)=0^610$, $\dots$, $\mu(9)=0^41001$. Given an ASCII message $m=m_1,\dots, m_n$ we define $\textbf{Encode}(m)=\textbf{Int}(\mu(m_1)\parallel \dots \parallel \mu(m_n))$.

Finally, for an ASCII message $m$ we can set
$$\textbf{Sign}_{sk}(m)=\textbf{Encode}(m)^d \mod N$$
$\textbf{Verify}_{pk}(m,\sigma)$ returns $1$ if and only if $\sigma^e=\textbf{Encode}(m)$.

Suppose Alice signs the message $m=$ Please pay Bob the following amount from my bank account (USD): 50. Suppose that Bob obtains $\sigma=\textbf{Sign}_{sk}(m)$. Explain how Bob can obtain a signature $\sigma'$ authorizing the bank to transfer more than $\$ 50$. How much money can Bob make? Assume that the Bank denies transfers above $750$ million (USD) without in person authorization. You may assume that $\textbf{Encode}(m)&lt;N/2^{64}$.

So I am not familiar with ASCII characters. But overall, I am not sure where to begin, nor which topic I should be referring to. Can I get some help started with this question?
If I read it correctly , just use the ASCII table, e.g. as given here. So "Please" is just the sequence of bytes (hexadecimal) 0x50  0x6c  0x65  0x61  0x73  0x65. Concatenation of those values gives a word (still hexadecimal, but we could have used bits as well, it's just more typing) 506c65617365 which is the decimal number $88426487575397$ (using a hex to decimal conversion tool, or bc in my case). This is the number which is raised to the power $d$ to create the signature of "Please".

As an idea to use $(xy)^k \bmod{N}  =x^k y^k \bmod{N}$ for all $x,y,k$, and of course $(x^d)^e = (x^e)^d = x  \bmod{N}$ for all $x$. This allows for manipulation in a multiplicative way for given signatures (or encrypted values).