It is intended that the tax offices will approach the taxpayers concerned in writing when processing the tax return.
The BMF letter is available here
For the declaration to be submitted to the tax office, the form Sample Application for Profit Generation Intention for Small Photovoltaic Systems and Combined Heat and Power Plants is available
In addition, the information sheet on the intention to generate profit with small photovoltaic systems is available in the section Forms - Income tax - Other
Further information can be found in the FAQ Taxes of Oberfinanzdirektion Karlsruhe under the heading "Photovoltaic systems and combined heat and powerplants".
The press release of the Ministry of Finance Baden Württemberg can be found here.