Okay, I will teach how to make a basic leveling system with no scripts. Please note that it is pretty time-consuming as you have to make a pretty massive amount of conditional branches, depending on the maximum level of the things you are implementing.
First, you will start with a Common Event. I named mine 'Normal Log Cutting' but you can name your event whatever is relevant to what you are making.
In my woodcutting system, There is a chance I can get no logs at all. This will get me no experience, so I will not add this after the End Branch. Instead, I will start adding the leveling system after I get logs successfully which is determined when the random number is greater than 5 and set the variables.
To set the variables, go to Control Variables and add a new variable. You should set it to add 1. My examples are 'LogsNeeded' and 'ChoppedLogs'. LogsNeeded will be the variable that keeps track of the amount of "experience" I have. As seen in the picture, when LogsNeeded equals 3, I level up to level 2 woodcutting. The reason I have 2 different variables (LogsNeeded and ChoppedLogs) is because I have more than one type of tree and the next tree after a normal tree (Oaks) will add 2 to LogsNeeded instead of 1, which means I get more experience from chopping Oaks.
The variable 'ChoppedLogs' is not needed but if you decide to add it, It will display the amount of total logs you chopped. If you want to add this, go to Control Variables and add a new variable. You can add this right under 'LogsNeeded'. You should also set this to add 1.
Under this, make a new Conditional Branch. Set it to the variable that matches my 'LogsNeeded' variable. For the value that it will equal, put however many "logs" you need to cut to get to level 2. Then, go to Control Variables and add a new variable for the level. I named mine 'Woodcutting Level'. Only when you level to level 2, make the variable add 2. For all the other levels, make the variable add 1. The reason for this is because the variable starts at 0, instead of 1. Under the line that says, Control Variables: [####: VARIABLE NAME] += 2, add text. Make it say:
You have leveled up in [WHATEVER]!
You are now level \v[THE VARIABLE NUMBER THAT STORES YOUR LEVEL]
Keep adding more Conditional Branches for every level, only this time change the value that LogsNeeded equals and switch WoodcuttingLevel to add 1 instead of add 2. To have a experience curve that gets harder as you level, multiply your LogsNeeded for level 2, with a calculator by a certain amount. I chose to multiply by 1.25. The higher the number, the harder the training. I wouldn't recommend anything over 2. Keep repeating this step for more levels. I hope this helped.