This sample code attaches EmailMessage to Account record with content of EmailTemplate

prepareEmailMessageList.java

public static List<EmailMessage> prepareEmailMessageList(List<Account> accList){
    List<EmailMessage> emailMsgList = new List<EmailMessage>();
    EmailTemplate templateObj = [SELECT Id,Name,Subject,Body,HtmlValue,Markup FROM EmailTemplate WHERE Name = 'Statement Template' LIMIT 1];

    for(Account accObj: accList){
        EmailMessage emailMsgObj = new EmailMessage();
        emailMsgObj.status = '3'; // email was sent
        emailMsgObj.relatedToId = accObj.Id;
        emailMsgObj.fromAddress = 'Billing.Support@acme.com'; // from address
        emailMsgObj.fromName = 'Billing Services'; // from name
        emailMsgObj.toAddress = accObj.Email;
        emailMsgObj.Subject = 'Your Statement is now available';

        List<Messaging.RenderEmailTemplateBodyResult> resList;
        resList = 
        Messaging.renderEmailTemplate(UserInfo.getUserId(), accObj.Id, new List<String>{templateObj.HtmlValue});
        emailMsgObj.HtmlBody = resList[0].getMergedBody();
        emailMsgList.add(emailMsgObj);
    }

    return emailMsgList;
}