DavidAgar and OldScaleGuy are correct. You want to turn the pump on and off with a switch and also slave the switch channel to the throttle so that the flow of oil is proportional to the throttle setting. At low throttle setting, the flow needs to be reduced to avoid killing the engine. According to Sullivan:

Set up notes for your Sullivan Skywriter Smoke Pump

The Sullivan Skywriter is a proportional device with a built in fail safe that
keeps it from turning on if the switch is accidentally in the wrong position when
your radio is activated. The Skywriter interprets the signal from your radio as
follows (the positive and negative values may be reversed for some radios).
ATV output from Receiver:

-140% to -15%: this is the desired signal when the switch is in the "off"
position. The Skywriter must see a signal in this range for at least 5 seconds
before it will activate.

0% to +40%: the Skywriter will not come on but this will not over-ride the
fail-safe, a negative signal is required for this (see above).

+41% to +100%: This is the proportional band of the control. The motor will
turn it's slowest at +41% and will be turning full speed based on the battery it has
at 100%.

+101% to +140%: This range is interpreted the same as +100%. The
skywriter will still run at full speed

Most people will slave a spare channel to the throttle then use a switch to
turn this function on and off. You need to set it up so that the when the switch is
off the output from the receiver is negative. (See sample program on next page)
Remember depending on the manufacturer of your radio the + and -
percentages could be reversed!

Some additional things to keep in mind:

***The Skywriter can operate at any voltage from 4.8 to 12V nominal. We do
however recommend that at battery of at least 6.0V be used until the pump head
is broken in (15-20 minutes of operation).

*** Sullivan also recommends the system is programmed and tested outside the
aircraft. At this time a substitute fluid can be used for testing such as nitro fuel or
water.