You can read a rather extended document I wrote below, which is still under development,
but first here is a summary with highlights and the important links:
0. Appreciate how awesome your ears are, and appreciate the sensory and emotional gains in listening to quality sound. This is cool: http://www.simonheather.co.uk/pages/articles/science_hearing.pdf
1. Work out your design. 2 or 3-way? Can you get an amplifier and crossover to match? Sealed or ported? The sealed vs ported question has many dimensions. Sealed is easier to make but will result in a larger box. Ported requires additional maths, and a more complex construction of a port into your box. Aside from this, do you prefer the sound of sealed or ported speakers? Sealed speakers are 'precise' and have much better impulse response, and give you the kind of drums you hear on an album like Dangerous, by Michael Jackson. Ported speakers have a slower response, so sound boomier and more washed out, giving a thicker sound. Ported speakers tend to achieve lower frequencies (<100 Hz) much easier because of the port design, but their frequency response is often highly peaked (tuned by the port length). You'll have a hard time maintaining a balanced frequency response (and keeping distortion low) at this low end, but the trade off is that you can achieve 30 Hz with a small (4 inch) driver and small enclosure. Personally I prefer sealed speakers!
2. Buy some drivers. For example - http://www.visaton.com/en/ or https://www.madisoundspeakerstore.com/speaker-drivers/
3. Calculate the enclosure volume required for your woofer - https://www.diyaudioandvideo.com/Calculator/SpeakerBoxEnclosure/
4. Meticulously plan out construction. This may help - https://www.diyaudioandvideo.com/Calculator/SpeakerBoxVolume/. You want to avoid any resonances (which isn't really possible), but you can certainly avoid making your enclosure square. Remember that any internal structure takes up volume, so make allowances for the part of the woofer that sticks into the enclosure, and any internal bracing. Fill your enclosure with a sound absorbent material like wall or loft insulation - why this helps is a little complex, as it only plays a minor role in damping internal resonances. As the woofer moves the internal pressure varies in the enclosure, and the material helps by absorbing and releasing heat, minimising the fractional pressure change in the box through the ideal gas law, and hence making it appear like the box is much larger.
5. Tuning your cross-overs and calibrating each driver. (I'm still developing this section, many plots and measurements to come!)
Final response measured! +- 3 dB most of the way, soooooo good!!!
Tip: Be wary of family members who don't approve of the dining table being used for speaker construction...