It is only complicated because its a new concept for you. No need for a professional.
Broken down for you.
1) determine the amp that will power the cab. Based on type of music, volume level, budget. Does not have to be a vintage or even a tube amp. Could even be a modeling amp.
2) figure out the impedance supported by the amp and wattage. Its all listed specs.
3) look for 12" speakers that match:
the style of music/guitar, another amp sound or are more neutral sounding to be more accommodating.
The impedance of the amp matches the speakers (listed in specs). Take into account wired either in series or parallel as I mentioned before to match. There is no discernable difference in sound serial vs. Parallel but just to reach desired impedance.
the wattage of the amp is supported by the speakers (listed in specs). The amp's specs (manual or website) should list the wattage for each particular impedance. In other words a change in impedance is a change in resistance so it also changes the wattage output of the amp. 4 ohms impedance will be a higher amp output than 8 or 16 ohms.
4) purchase the speakers!
5) open the cabinet's back and remove (if you want) the existing speaker.
6) install the new speakers (its just some washers and nuts
7) get some 12 gauge wire and if needed a new input jack (less than $10) and wire the speakers to the input jack which is installed on the back, also just a washer & nut. You do need to wire in serial or parallel. Serial means positive (center of input jack ) to positive of speaker 1; negative of speaker 1 to positive of speaker 2; negative of speaker 2 to negative (outer of input jack) on jack.
Parallel means positive of input jack is connected to positive of both speaker 1 & 2; negative of both speaker 1 & 2 connected to negative of input jack.
You do need to solder the connections which requires a $10 soldering iron. Real, real easy to.learn to do. Takes less than five minutes for a newbie. Good knowledge to have to work on instruments as well!
8) close up the back of the cabinet
9) plug speaker cable into the amp & cabinet
10) play guitar or bass!!!!
Avoid the youtube videos! I have seen many that give wrong info.