At it's most simple OnStep needs just a few components to make a working controller. The image below I took during early testing.
The Microcontroller is an Arduino Mega2560, this has Bluetooth wired up too (an HC05.) The red/black jumpers from the Mega2560 carry power (+5V/Gnd) over to the HC05. The teal/yellow jumpers carry the Mega2560's Serial1 RX/TX to the HC05's TX/RX (never run TX to another device's TX!)
The stepper driver is a Sparkfun Big Easy Driver (BED) and so there's just one Axis (RA) working. Power for the stepper motor is wired into the Breadboard (not shown) and comes out on a pair of jumper wires and goes into the BED (Vmot/Gnd.) The four pin ribbon cable carries signals for Step/Dir and logic Gnd (one wire isn't connected) from the Mega2560 to the BED.
Add one more stepper driver (plugged in similar to the RA driver) and stepper motor and this could be a telescope controller.
Nowadays I wouldn't recommend using the Mega2560 (unless low cost is major priority) or BED but the same concept applies to the Teensy3.2 and there are cheap "breakout boards" on eBay that you can drop a Pololu format stepper driver into and plug stuff right in.
Note the Mega2560 uses 5V logic level signaling; I thought at the time the HC05 I bought was designed for 5V logic levels but in-fact they're all 3.3V logic (but 5V power, AFAIK) so this setup wasn't ok!
The expensive Sparkfun Bluetooth Mate Silver can run at 3.3V (3.3V logic) or 5V (5V logic.)
The ESP-01 WiFi runs at 3.3V and handles both 3.3V or 5V logic levels.
The HC05/HC06 runs at 5V and does only 3.3V logic levels.
OnStep can use different "pin maps" and the following was designed for the Config.Classic.h wiring!
Not shown here but whatever you build use an appropriate DC rated fuse (current rating depends on stepper drivers/motors/etc.)