How to forge a textbook RSA signature?
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).