Now you are talking about 2nd edition rules. ^^ But the 5e multiclassing system is both easier to use, and more effective and not restricted to specific races. If you are unfamiliar with how it works in 5e there are just two simple things to keep track of. 1. Every class levels at the same rate, rogues dont level faster than others and so on. 2. When you have enough XP to level up you can take the new level in any class in any order. You never loose any old class features while leveling up a new class, and you dont "leave" a class behind when starting a new one. You can start as lv1 fihter, then pick up a level as rogue, and then go back and take another level of fighter.
To figure out what stuff your character has you just total the stuff from each class you have taken, so if you do as the example above you would have a lv3 character that has the abilities of a lv2 fighter + a lv1 rogue. (it is slightly more complex but that is a close aproximation of how it works.)