Composing second species counterpoint in the 18th century style of J.S. Bach (1685-1750) is a challenging but important skill to master.  If you look at  Bach's Two Part Inventions you will discover that most of the counterpoint is really in second species - a term denoting a rhythmic relationship of two notes in one voice against one in the other.

    The music in Ex. 1 is a typical example of second species (or 2:1) counterpoint in the Bach style.  It is in the key of Eminor and notice that the leading tone (D#) is raised when it occurs.

    Example 2 however has a number of problems in it.  The most obvious being the parallel octaves in bar 1.  But equally problematic are all of the leaps in the treble part.  An important principle in counterpoint is to use mostly stepwise motion.  Which of the two examples above would you choose to sightsing on a test?  Although all of the leaps are using only chord tones there are simply too many.
    In two spots there are leaps to or from notes which, although chord tones, form a Perfect 4th interval with the C.F. (cantus firmus - or given part).  Perfect 4ths despite their name are actually dissonant intervals and should only be used as non-chord tones (NCT's) in 18th century counterpoint, so leaping into or out of them is not so good.  In bar two of Ex. 2 had the final note of the measure been an 'A' the P4 on beat 4 would be OK, because it would be a type of appogiatura resolving by step downwards.

    Example 3 below is just like Example 1 - but analyzed with Roman numerals and the NCT's labelled.

    In Example 3 the use of a number of passing tones minimizes the leaps found in example 2.  Of the various types of NCT's you should greatly favor passing tones and neighboring tones since these are the only NCT's that do not involve leaps.  While passing tones can be both unaccented (pt) or accented (apt), the unaccented are more common.  Be wary of using too many APT's - in general chord tones should fall on the accented part of the beat, and you should avoid having accented NCT's on two consecutive beats.
    Example 3 also incorporates an appogiatura (app) in bar 2, beat 3.  Appogiaturas are very effective NCT's, but should be used sparingly, and they are far more common in their ascending form as above, with the descending form being much less frequently found in 18th century music.  The appogiatura above also uses the striking melodic interval of a diminished 7th (the green circled notes).  While leaps of sevenths are not frequent, the dim 7th is in fact quite common in Baroque era music - it is in fact a "signature sound" of the style.  Diminished intervals are OK to use as melodic intervals if and only if the next melody note is a stepwise resolution in the direction opposite the leap - but their inversion, augmented intervals are not permitted.   You should also notice that the ascending leap of a P5 at bar 1, beat 3 is filled in by steps down from the upper note of the leap.