When the Messages app is opened, it will begin as you last left it. It will probably either be displaying messages in a conversation you have had with one person or it will be showing a list of all your current conversations, typically with several people. An expanded conversation with one person will be headed by the person's name or number. The list of conversationss has a Messages heading above the list.
If the Messages app starts in the main list of conversations, VoiceOver will probably begin with its focus on an item near the top of the screen. You can swipe right to move past the Messages heading and reach the first item in the list of conversations. This will be the conversation with the most recent message. The message will be spoken in full by VoiceOver but you can pause VoiceOver speech with a two finger tap or swipe right to hear the most recent message in the next conversation while the earlier message is being spoken. If you continue to swipe right through the list you will hear the most recent message in each conversation, which may be either from you or your correspondent. If you want to hear earlier messages in a conversation, activate the conversation by double tapping and VoiceOver will speak the most recent message in the conversation which will be at the bottom of the list. you will then be able to swipe left back through earlier messages. You may sometimes need to scroll with a three finger swipe down. There will be a back, button at top left of the screen to take you back to the main message list. Alternatively you can go back with a two finger scrub gesture.
If the Messages app starts in an expanded conversation, then the VoiceOver focus will probably be on the back, button and you can double tap to return to the main message list if you wish. Alternatively, you can explore the conversation that is on the screen by swiping right through it.
If you want to reply to a message, locate it in the main messages list and then double tap to activate the conversation. Feel near the bottom of the screen for the text field in which you can write a reply. You will hear either "message, imessage, text field" or "message, text message, text field", depending on whether the recipient uses an Apple device or not. Double tap and you should hear something including "text field, is editing". You can now type, dictate or handwrite your message. Finally feel above the top right corner of the keyboard for the send, button and double tap. You should hear a whoosh to indicate that the message is sent. If you dictated the message, then you can proceed to the Send button with a swipe right. Note that there is no cancel button to be used if you have second thoughts and decide not to send the message. If you wish to delete your reply, you can either shake the phone to undo typing or turn the rotor to Edit, swipe up to hear "select all" and then double tap to select all the text. Finally, use the delete key on the keyboard to delete the text.
All the messages in a conversation can be deleted from the main messages list.
Locate the conversation you want to delete by finding the most recent message in the messages list and then swipe up or down until you hear "delete" and then double tap. You will hear "Would you like to delete this conversation?";you should now swipe right to find the "Delete, button" and then double tap to confirm the deletion.
It is also possible to delete individual messages. Open the conversation containing the message you want to delete and locate the message to be deleted. Swipe up until you hear "more" and then double tap. You will hear "selected", followed by the message. Locate the toolbar at the bottom of the screen . It has two buttons; there is a delete button to the left and a forward button to the right. Locate the delete button and double tap. Finally confirm that you want to delete the message.
See Forwarding messages using the Messages app
Note that swiping up or down in the messages list can also find the "hide alerts" feature. I don't recommend using this. It is designed to stop you receiving new message alerts from an individual who is sending you an unacceptable number of messages. If you use this accidentally, it is easy to revert. Find a message from the individual that you have accidentally hidden alerts from. VoiceOver will speak the word muted after their name. Swipe down until you hear "show alerts" and double tap.