Split-levels are a bear to keep uniform.
What I did about 20 years ago was take a regular lamp timer and did some surgery on it to separate the on-off switching from the electricity that powers the timer. In other words, I converted the timer into a “dry” switch. I connected the “dry” switch in parallel with the low-voltage thermostat wiring that controls the fan. I set mine up at the box on the furnace that the thermostat wire runs into. “Plan B” would be to run a wire to the thermostat and connect the switch to the R and G terminals - this might work out to be an easier option if your furnace is in the attic or some similar hard to reach location.
Usually, this will be the “R” and “G” terminals, and usually the wire colors are red and green. No promises here - thermostat wiring can vary from one brand to another, and whether or not the original installer bothered to follow the normal color scheme. If you’re unsure, you might want to call a heating contractor - miswiring this will result in anything from nothing happening to damaging the thermostat or even damaging the furnace itself.
This will give you a blower control that’s purely time-driven. You’ll need to figure out what time(s) of day you want the blower to run and set the timer accordingly.
If you want to have the blower controlled by temperature, you’ll need a second thermostat. For this, get an old-style non-powered, non-setback heating and cooling model - you just need a dry switch contact here. Set this thermostat to “cool” and connect the R and Y terminals to the blower control’s R and G terminals. “Y” (yellow wire) is the cooling signal wire - when the temperature exceeds what the termostat is set for, it will make a contact between R and Y.
However you do it, it can be a very worthy venture both in terms of comfort and energy efficiency. The upstairs south-facing bedrooms acted as passive solar collectors and would get uncomfortably warm. By circulating the air, we were able to keep the rooms much more comfortable and there was a resulting energy savings - before doing this, the heat would come on immediately when the setback program called for heating. By starting up the blower half an hour before this time, there was usually a delay of up to an hour before the burner came on.