How to create a Approval Workflow in SharePoint Designer ?

A high level answer to your question -

Open your SharePoint Designer (SPD) and then connect to your SharePoint (SP) site
Open open, go to "Workflows" section on the "Site Objects" (quick launch section)
Here you'll get option of creating workflows of the following types - List Workflow, Reusable Workflow, and Site Workflow. Select the one suitable for you.
Once you select any of the above, you'll be prompted to give details for the Name and Description of the workflow. Fill these and click on "OK" to go to the Workflow editor screen
Design your workflow logic using the tools (Condition, Action, Step, etc). You can find these in the top ribbon bar.
Click on "Save" and "Publish" to complete your workflow deployment in SharePoint

There you go! Your wofklow is ready!