Calibrating a multimeter without another multimeter
If accuracy is in any way important to you, you will have to spend the money and get a decent multimeter. Depending on how much you want to spend you can get anything between 0.1% and 0.000001% accuracy. What is it worth to you ?

Also ask yourself: what does it bring me if I know this battery is 1.49 V as compared to knowing it's 1.4936 V ? Do you need that accuracy ? If so for what ?

I would forget about trying to calibrate that old multimeter, chances are you will make things worse than they are now. Instead do some tests on it (measure voltage, resistance) and compare the result to that of a know-to-be -accurate meter. If the old meter is in the same ballpark, just use it for the things where you don't need a high accuracy.