Simplest Approach:
For Custom Error Pages - we can use Site Pages / Layouts Pages (Recommended)
Browse to C:\inetpub\wwwroot\wss\VirtualDirectories\ and open the folder associated to the Share Point site you would like to affect.
Edit the file web.config and find the keyword custom Errors. Replace the complete custom Errors code block with:
Web.Config Entries:<customErrors defaultRedirect="ErrorPagePath" mode="On"/>ErrorPagePath can be Layout page path i.e "_layouts/Folder Name/Page Name.aspx"
or
ErrorPagePath can be Site Page path i.e."site pages/errorpage.aspx"
site pages can be created using SPD.
User can drop the web part according to their customization.
