Go to the App Store and download the Font Maker App (it's the First one listed)
Once you've downloaded the app, open it up onto this page (it's the first page it opens up to)
Now select the "a" button
You will see these letters pop up
Now, choose which letter you want. I have chosen the "@" symbol, and this will be used for all letter "a's" in your new font
Now, notice that down the bottom the letters you have chosen to replace the other letters have popped up
Continue doing this for the rest of the alphabet
Then, when your finished, tap "Save" and press "OK"
Now, down the bottom you will see a button saying "Use Fonts". Click on this
Then select your font from the top section (it is the third one down)
Then, start typing whatever it is that you want to type
When you are done typing, click "Copy to Clipboard"
Then go into notes, messages, whatever app you want to type your new font into and click paste
And there you go, your new font!