3rd Species
In third species you add four notes to each one in the cantus firmus (i.e. crotchets against semibreves). At least the first note, and either the third or fourth must be consonant. Dissonances must be part of a linear movement between consonances.
Use a search engine or follow one of the links to find detailed instructions about how to write 3rd species counterpoint.
Chicago University counterpoint site
Stanford University counterpoint site