The steps for opening a perspective in its own window are identical to opening a perspective in the same window with one minor difference. As you are choosing the perspective (either from the Open Perspective button on the shortcut bar or the Open > Perspective menu) hold down the shift key.
By default, selecting a perspective to open will open it in the same window, and holding shift will open it in a new window. You can configure this default behavior using Window > Preferences > Workbench.