Tuning the Transmitters
You may have noticed each astable 555 timer circuit includes a potentiometer. This allows us to fine-tune the frequency at which the circuit oscillates. We want it to be as close to 38kHz as possible so the receiver can see it in full strength.
This is where an oscilloscope comes in handy. This nice piece of kit lets you visualize voltage waveforms in (pretty much) real time. My current oscilloscope is a USB-based one called the Analog Discovery 2 by Digilent. This device not only acts as a 2-channel oscilloscope but also has a 2-channel function generator, a variable power supply, a pattern generator for digital circuits, and a logic analyzer! I prefer actual lab equipment but the Analog Discovery is nice for how compact and affordable it is.
Use a small screwdriver to adjust the potentiometer. When you have it tuned just right, you may want to hot glue the knob in place so it doesn't get changed accidentally. Make sure to repeat this procedure for each transmitter circuit!
Tuning Without an Oscilloscope
To tune the circuit without an oscilloscope, you can just play with the potentiometer until the circuit operates reliably. The IR receiver will work with frequencies that are close enough to 38kHz.
Tuning With an Oscilloscope
If you are lucky enough to have access to an oscilloscope, this part becomes very easy. Simply hook your probe up to the output pin of the astable 555 timer, and the waveform should appear. You can adjust the potentiometer until the frequency is about 38kHz.
With the equipment mentioned above, measure the frequency of the wave using vertical cursors, and place them one period apart. The inverse of the difference between them will be your frequency (the oscilloscope usually does this calculation for you).
If you're just getting started with the oscilloscope, you should read this guide.