OK, we will begin by discussing voicing. You might know all of this already. As soon as you finish this we will begin voice leading chords in four part harmony. Since you say you cannot fluently determine intervals on a piano staff, these exercises are very important. If you cannot find parallel fifths at a glance, you can't avoid them. We will mostly be working with 4-part keyboard harmony.
Chords
You probably know what a triad is: a harmony consisting of three notes, in this style, separated by thirds. So in a triad there is a root note, a third, and a fifth (which is two stacked thirds). In actual chorale style harmony, the root note is often repeated, giving you a chord of four notes. These four notes may be moved into any octave, so that the 3rd becomes a 10th, the 5th a 12th, the unison an octave or 15th. I hope you know all the names of the intervals (like unison, second, etc...).
Voicing
In chorale style harmony, the notes of the triad are arranged in a certain way to make the most balanced sound. Usually the whole chord is not very far outside of the treble or bass clefs' ranges.
To begin with, we will work with chords whose roots are in the bass, or as I call them, 5 or 5/3 chords. For now we will only use chords with a single third and fifth, and a doubled (repeated) root (usually in another octave).
Open the attached file and list out the intervals each note of each chord forms from the bass, from the bottom up. Ignore the notes without a chord for now. For example, the first chord is 5, 8, 3. You notice I wrote 3 instead of 10, because that note functions as the third. And it will be easier for all of us

. Try to be as fast and accurate as possible.
Now list the intervals between successive notes of those same chords, that is, the interval between each note and the next note above it. For the first chord, 5, 4, 3.
Now I will explain those chords. You probably know this too; I hope this isn't too basic. A balanced chord's notes are never separated from each other by more than an octave, except from bass to tenor. Thus from soprano to alto, and from alto to tenor there should be less than an octave. Tenor to bass, however, be more than or less than an octave.
Note that tenor to soprano can be over an octave. When it is, you have an open disposition. When tenor to soprano is less than an octave, you have a closed disposition.
Notice the difference of sound (listen to my example) between open and closed chords.
Now tell me whether each of my chords is open or closed, and if it follows all the rules.
Then, make your own chords on each of the roots I have written, without using any accidentals. Remember, 3rd, 5th, and doubled root. Make the first two closed and the last three open.
If this is way too basic, just tell me and we'll skip it all. If not, then you should follow it all exactly. Do not break any rules unless you have mastered them. So, for example, in 4 part counterpoint, upper voices might be separated by over an octave, but such an arrangement is usually very brief. There is no reason for there to be any mistakes in your answers (unless, like I said, this is too basic. Then we can skip all of this). You need to have fast interval identifying skills to write progressions and avoid parallels.