To forward messages, type `f' or `F' in Summary mode. Then, Draft mode appears and the messages are already attached to the attachments region.
Also, you can prepare the attachments region by yourself, then copy(`c') the messages or make links(`l') to the messages. If the file name of the messages is numeric([0-9]+), they are automatically considerd as messages. `y' is very convenient because it make a link to the message displayed in Message mode.
By default, the entire message is forwarded. If you want to remove some parts of its header, define `mew-field-delete-for-forwarding'. The following is an example to remove "Received:" and "Return-Path:" when forwared.
(setq mew-field-delete-for-forwarding '("Received:" "Return-Path:"))