After you have registered your number with Signal, you can start communicating privately with other Signal users. 

Here's how to send a message:

• Android
• iOS
• Desktop

Android

1. Open Signal and tap on the compose icon  to view your Signal contact list.
2. Select a contact or enter a number to open that conversation.
3. Tap on the area where you type a message.
• You will see "Signal message" when you are securely communicating with another Signal user. 
   
• You will see "Unsecured SMS" when you are sending insecure messages through your mobile plan.
   
4. Type your message or attach a file.
5. The send icon also indicates the message type.
• To send a Signal message, tap the blue send icon with a closed lock.
  
• To send an insecure SMS/MMS through your mobile plan, tap or long-press the send icon to switch to the gray-icon.
    

iOS

1. Open Signal and tap on the compose icon  to view your Signal contact list.
2. Select a contact or enter a number to open that conversation.
3. Tap on the 'New Message' area to type a message.
   
4. Type your message or attach a file.
5. Tap Send. 

Desktop

1. In Signal, scroll through your Signal contact list or click on the search field: 
   

2. Enter your group name, contact name, or the full phone number.
3. Click on the group or contact, or choose 'Start conversation...' after manually entering a phone number.
4. Your conversation will appear on the right side of your contact list.
5. Click on the 'Send a message' area to type a message.
   
6. To send, press Enter.