Random substitution ciphers are subsitution ciphers that change letters for symbols. This, for example, could be your cipher...

...where "A" (plaintext) becomes "" (ciphertext).

So, using that, "Amelia likes cheese, but not your face" becomes...

NOTE: This is just the font Wingdings 3 in all capital letters. It's best if you make up your own code, so that it's harder to crack.

A very popular random substitution cipher is the Freemasons' Cipher (also "Masons' Cipher" or "Pigpen Cipher")...

Another popular one is the Rosicrucian Cipher...

...where the letter in the box is replaced with a dot like so...