Find the user whom you want to send the message. Click on his avatar or name. In some forums you need to click left and right mouse button and from the context menu select "user Profile". You will be automatically redirected to its page.
On the profile page next to your account picture click on send message. It can be marked with the envelope icon, "dogs", the letters "BOS". It may be the inscription: "Send a message", "message", "Email user" or similar. In English-speaking resources looking for similar translations. Click on this button.
In the "Subject" field enter the question you want to ask. On most forums this field is optional, but if the user doesn't know you, it is better to specify that the message wasn't spam.

Under subject, enter the text of the message. Even lower can press the button "Attach" ("Attach", "Attach") and add a small file of any format (video, photo, audio, text).
Click "preview" to check the message. Then "Send" that message was delivered.