SharePoint 2013: http://blogs.msmvps.com/windsor/2011/11/04/walkthrough-creating-a-custom-asp-net-asmx-web-service-in-sharepoint-2010/
SharePoint 2007: Create ASP.NET Web Service Application
and follow the following blogs in case of this issue: https://dhondiyals.wordpress.com/2011/02/10/resolved-could-not-load-file-or-assembly-microsoft-sharepoint-search-version12-0-0-0-cultureneutral-publickeytoken71e9bce111e9429c-or-one-of-its-dependencies/
