Get the sheet music for the Pokemon song you want to play (see Resources).. If commercial sheet music is not available, you can transcribe the music yourself onto manuscript paper if you have a good musical ear and music transcription skills. Otherwise, get someone else to help you transcribe the song, working out the note pitches in the order they occur in the song. Alternatively, music transcription software is available that will transcribe the music automatically and provide you with sheet music showing the pitches and rhythms of the song.
Work out the recorder fingering for each note pitch, using a recorder fingering chart (see Resources.) Practice playing through all the song's note pitches in order, until you have the sequence of recorder fingering patterns memorized. Work slowly and do not expect to be able to play the entire song on the first attempt--if you find it difficult to play the notes in order, work on small sequences of two to three notes, then gradually string smaller sequences together into longer sections of the song. This may take several practice sessions over a few days.
Play along with the recorded version of the Pokemon song. This will help you learn the rhythm of the song (how long to hold each note). You will probably notice patterns of both rhythm and pitch that repeat in the song. Practice playing along with the recording until you have the song's pitches, fingering positions and pitches memorized.
Perform your Pokemon song on the recorder as a solo, without the recording playing. Bravo!