Make sure the object has UVs and a Material assigned to it.
You also need to have a texture projected onto a model in order to use the UV Pelt tool. This tool allows you to manually select the boundaries of the UVs where you want the surface to unwrap.
Using UV Pelt
Click the UV Pelt tool on the Texture tab.
Select the edges that will form the boundary of the pelt.
You can easily select loops of edges using the Edge Loop tool.
Press Enter to unwrap your selection.
For specific parameter help see the UV Pelt node help.