You can select the alternate function of every pin using the GPIOx_AFRL and GPIOx_AFRH, with the help of the alternate function mapping in the datasheet you can see which pin is able to do which function (not all pins can be used as DCMI pin). Note that most of the functions are available on multiple pins and you should select only one pin for each alternate function.
For DCMI you have to use AF13.
As noted in the reference manual you must only select the alternate function for the data pins which are really used (D0-D7 for this camera).
But just selecting the alternate function is not enough. You also have to switch the pin to the alternate function using the GPIOx_MODER (writing 0b10 to the correct bits).
As DCMI has only inputs, the below is just additional information if you encounter alternate function outputs for something else:
Depending on the specification of the bus, you also have to set the output type to open-drain or push-pull using the GPIOx_OTYPER.
And finally the output speed must be selected, so that the pin can drive the outputs fast enough using the GPIOx_OSPEEDR.