It is envisaged that the tax offices will write to the taxpayers concerned when processing the tax return.
The BMF letter is available here
For the declaration to be submitted to the tax office, the form Antragformular Gewinnzielungsabsicht bei kleinen Photovoltaikanlagen is available
In addition, the information sheet on the intention to realise a profit with small photovoltaic systems is available in the section Forms - Income Tax - Other
Further information can be found in the FAQ Taxes of the Oberfinanzdirektion Karlsruhe under the heading"Photovoltaic systems and combined heat and power units".
The press release of the Ministry of Finance Baden Württemberg can be found here.