How to associate a workflow to a list programatically

Associating workflows to lists are pretty simple.

You will need the following:

Task List - Out of the box list
WorkFlowHistory List - Out of the box list
Workflow List - Workflow associate to this list

SPSecurity.RunWithElevatedPrivileges(delegate()
{
web.AllowUnsafeUpdates = true;

SPList taskList;
SPList workflowHistoryList;
SPList indexItemList = web.Lists[Constants.IndexItemList];

try
{
taskList = web.Lists[Constants.WFTasks];
}
catch
{
taskList = web.Lists[web.Lists.Add(Constants.WFTasks, Constants.WFTasks, SPListTemplateType.Tasks)];
}

try
{
workflowHistoryList = web.Lists[Constants.WFHistory];
}
catch
{
workflowHistoryList = web.Lists[web.Lists.Add(Constants.WFHistory, Constants.WFHistory, SPListTemplateType.WorkflowHistory)];
}

SPWorkflowTemplate template = web.WorkflowTemplates.GetTemplateByBaseID(new Guid(Constants.workFlowTemplateId));

SPWorkflowAssociation association = SPWorkflowAssociation.CreateListAssociation(template, template.Name, taskList, workflowHistoryList);
association.AllowManual = true;
association.AutoStartChange = true;
association.AutoStartCreate = true;

indexItemList.WorkflowAssociations.Add(association);
indexItemList.Update();

web.AllowUnsafeUpdates = false;
web.Update();
});

Workflow start properties may vary, please change as you see fit.