Define a flexform for your plugin

ext_localconf.php

<?php

//...
/**
 * The partners plugin
 */
\TYPO3\CMS\Extbase\Utility\ExtensionUtility::configurePlugin(
	'Vendor.' . $_EXTKEY,
	'Partners',
	array(
		'Organisation' => 'list, show, search, listByIndustry, listByService',
	),
	// non-cacheable actions
	array(
		'Organisation' => 'search, listByIndustry, listByService',
	)
);

ext_tables.php

<?php

/**
 * The partners plugin renders all Organisations
 */
\TYPO3\CMS\Extbase\Utility\ExtensionUtility::registerPlugin(
	$_EXTKEY,
	'Partners',
	'Vendor: Partners',
		\TYPO3\CMS\Core\Utility\ExtensionManagementUtility::extRelPath($_EXTKEY) .
		'Resources/Public/Icons/Plugins/plugin.partners.gif'
);

//..


/**
 * Generate an adequate pluginSignature
 */
$extensionName = \TYPO3\CMS\Core\Utility\GeneralUtility::underscoredToUpperCamelCase($_EXTKEY);
$partnersPluginSignature = strtolower($extensionName) . '_' . strtolower('Partners');

/**
 * Register FlexForm for Plugin Partners
 */
$TCA['tt_content']['types']['list']['subtypes_addlist'][$partnersPluginSignature] = 'pi_flexform';
\TYPO3\CMS\Core\Utility\ExtensionManagementUtility::addPiFlexFormValue(
  $partnersPluginSignature,
		'FILE:EXT:' . $_EXTKEY . '/Configuration/FlexForms/Partners.xml'
);

Partners.xml

<T3DataStructure>
    <sheets>
        <sDEF>
            <ROOT>
                <TCEforms>
                    <sheetTitle>Behaviour</sheetTitle>
                </TCEforms>
                <type>array</type>
                <el>
                    <switchableControllerActions>
                        <TCEforms>
                            <label>Display Type</label>
                            <config>
                                <type>select</type>
                                <items>
                                    <numIndex index="0">
                                        <numIndex index="0">Show Search Form</numIndex>
                                        <numIndex index="1">Organisation->list;Organisation->show;Organisation->search</numIndex>
                                    </numIndex>
                                </items>
                            </config>
                        </TCEforms>
                    </switchableControllerActions>
                    <settings.partnerFinderPage>
                        <TCEforms>
                            <label>Target page for the partner finder</label>
                            <config>
                                <type>group</type>
                                <internal_type>db</internal_type>
                                <allowed>pages</allowed>
                                <size>1</size>
                                <maxitems>1</maxitems>
                                <minitems>0</minitems>
                                <show_thumbs>1</show_thumbs>
                            </config>
                        </TCEforms>
                    </settings.partnerFinderPage>
                    <settings.partnerFinderPageSingle>
                        <TCEforms>
                            <label>Target page for the partner finder (Single Partner)</label>
                            <config>
                                <type>group</type>
                                <internal_type>db</internal_type>
                                <allowed>pages</allowed>
                                <size>1</size>
                                <maxitems>1</maxitems>
                                <minitems>0</minitems>
                                <show_thumbs>1</show_thumbs>
                            </config>
                        </TCEforms>
                    </settings.partnerFinderPageSingle>
                    <settings.casestudy.page>
                        <TCEforms>
                            <label>Target page for success stories plugin</label>
                            <config>
                                <type>group</type>
                                <internal_type>db</internal_type>
                                <allowed>pages</allowed>
                                <size>1</size>
                                <maxitems>1</maxitems>
                                <minitems>0</minitems>
                                <show_thumbs>1</show_thumbs>
                            </config>
                        </TCEforms>
                    </settings.casestudy.page>
                    <settings.service.page>
                        <TCEforms>
                            <label>Target page for the services plugin</label>
                            <config>
                                <type>group</type>
                                <internal_type>db</internal_type>
                                <allowed>pages</allowed>
                                <size>1</size>
                                <maxitems>1</maxitems>
                                <minitems>0</minitems>
                                <show_thumbs>1</show_thumbs>
                            </config>
                        </TCEforms>
                    </settings.service.page>
                </el>
            </ROOT>
        </sDEF>
        <target>
            <ROOT>
                <TCEforms>
                    <sheetTitle>Tag Cloud</sheetTitle>
                </TCEforms>
                <type>array</type>
                <el>
                    <settings.header>
                        <TCEforms>
                            <label>Heading for the Plugin</label>
                            <config>
                                <type>input</type>
                                <eval>trim</eval>
                                <size>60</size>
                                <default></default>
                            </config>
                        </TCEforms>
                    </settings.header>
                    <settings.industryShowPage>
                        <TCEforms>
                            <exclude>1</exclude>
                            <label>Target page for single Industries</label>
                            <config>
                                <type>group</type>
                                <internal_type>db</internal_type>
                                <allowed>pages</allowed>
                                <size>1</size>
                                <maxitems>1</maxitems>
                                <minitems>0</minitems>
                                <show_thumbs>1</show_thumbs>
                            </config>
                        </TCEforms>
                    </settings.industryShowPage>
                </el>
            </ROOT>
        </target>
    </sheets>
</T3DataStructure>