Hi,
U made a tank. u would like to convert it.

Here's how u can do it. open the solid part ( the tank u made). I think it might be a cube with top surface open by shell option.
Click Applications from ur menu bar. Select sheetmetal in the drop down list. from the menu manager select Driving srf (since u already made shell dont go for shell option in the menu manager). It prompts u to select surface to be used as driving sheet metal surface, U select the bottom surface. Enter the desired thickness. your GUI will now be changed to sheetmetal mode.
Now click insert from menu bar, select conversion. SMT CONVERSION box will open. select edge rip and click define. select the 4 outer edges which connects the side walls. click done sets. U can define the corner relief if u want. or else click OK. u can now see the ripped edges.
Now u can use flat pattern to unfold the part.

Hope it'll be helpful. If any other information needed please let me know
Mail id: ezcayster@gmail.com