I start with an out of the box team site.

Open SharePoint Designer 2010

I go to Site Actions Edit in SharePoint Designer this way I know I'm connected to the correct site.

Click the File Tab

Within SharePoint Designer 2010 I click the File tab.

Click More Pages

I click More Pages. Choose aspx page and click create

This will create an out of the box .aspx page which knows nothing about SharePoint.

Give the page a Title and location for storage

I will place my page into the Site Pages Library.

Choose Advance Mode (or not)

I will choose advanced mode. This is one of the ways a SharePoint item can be 'customized' meaning that it will be stored in the Content Database in as opposed to using the site definition in the 14 folder.

Delete the Form Tag

I normally hit the escape key to highlight the default form tag then I hit delete to delete it.

Click the Style Tab

I click the Style tab.

Attach the Master Page (I'll choose default)

I click the Attach drop down arrow and choose my Master Page. I'll choose the Default master page, which in this case is the v4.master.

Put the Content Place Holder Main into Custom Content Mode

The Carrot is sometimes hard to find and it's important that you choose the correct content placeholder here (Main).

Add Content

Add some custom content inside the PlaceHolderMain(Custom)

Save The Page

Save the Page and Preview in the Browser! All done. That's how you add a custom .aspx page to your SharePoint Team site.