Measure the distance between the two posts where you will be installing the slats. Write down, or remember, this measurement.
Decide how many slats you will be installing. Consider that the building code in most places calls for the distance between slats on railings to be no farther than 4 inches apart. If you are installing the slats on something other than a railing feel free to vary the distances.
Measure the width of a slat and add that measurement to the measurement of the span between the two posts.
Divide your new measurement by one plus the total number of slats. For example, if you are putting six slats in a span, divide by seven in your calculations. This will give you the distance from the edge of the post to the opposite side of the slat first slat.
Find the distance between the slats by subtracting the width of the slat from the previous measurement. This distance is the space between each slat for equal spacing.
Cut a wood block the length needed for equal spacing and use the block as a spacer to install the slats.