http://office.microsoft.com/en-us/sharepoint-designer-help/video-create-an-approval-workflow-in-sharepoint-designer-2010-VA101897477.aspx

http://plexhosted.com/billing/knowledgebase/226/How-to-create-a-simple-approval-workflow-in-SharePoint-Designer.html

SharePoint – Custom approval workflow with Dynamic Approvers using SharePoint Designer

http://amalhashim.wordpress.com/2013/03/14/sharepoint-custom-approval-workflow-with-dynamic-approvers-using-sharepoint-designer/

5 Steps to Enhance SharePoint 2010 Approval Workflow

https://www.nothingbutsharepoint.com/sites/eusp/Pages/5-Steps-to-Enhance-SharePoint-2010-Approval-Workflow.aspx

5 Limitations of SharePoint Approval Workflow

https://www.nothingbutsharepoint.com/sites/eusp/Pages/5-Limitations-of-SharePoint-Approval-Workflow.aspx

Creating Custom Workflow Task Approval Forms with SharePoint Designer 2010

http://blogs.msdn.com/b/edhild/archive/2011/06/01/creating-custom-workflow-task-approval-forms-with-sharepoint-designer-2010.aspx