Forms Designer for SharePoint 2010 installation guide
Download setup file and run it on one of the servers in your Sharepoint 2010 farm as Farm Administrator. Follow wizard steps.
Go to your application. Select Site Settings item in the root of the site collection in Site Actions menu. Choose Site collection features in Site Collection Administration section:
Activate Forms Designer feature.
Now select a list or a library in the current site collection and choose List or Library tab in the ribbon. Here you can find a new Design Form button in Customize List section: