Marshmallow: for the cough and irritation (expectorant and demulcent); with
Yarrow: for the fever and cough (diaphoretic and expectorant); and
Peppermint: to improve the flavor of the mixture. The antispasmodic action would assist the cough.
I would add equal parts to a glass tea cup. Pour boiling water over herbs and let steep 5-10 minutes. Strain and enjoy.