[Shasown]

Easy way is to go to a message from whoever you want to write to then place your cursor over the message, this should highlight it and over at the right hand side you will see a star over an arrow. Click on the arrow a message box will open near the top of the message list with @whoever as the start, type your message and send.

the number that appears at the top right of the message box tells you how many characters you can type in your message.

Or in the box at the top marked what you are doing type "@name" (without quotation marks) this changes it to a reply box as above. make sure you type their name correctly including any punctuation marks etc.