How to avoid overexposure
The important thing to bear in mind when you’re shooting images for a multiple exposure is that each frame needs to be underexposed, to prevent the final merged image being rendered too bright.
Your camera can handle this for you, based on the number of shots you’ve specified for your multiple exposure. If you want to do it yourself, though, you’ll need to dial in negative compensation for each shot. The rule of thumb is to use half the number of shots, measured in stops. So, for two exposures, you’d dial in -1 stop for each shot, and for three exposures you’d use -1.5 stops. It’s only a general guideline, though: you may find that you prefer the end result slightly brighter or darker than the ‘standard’ exposure. It’s worth doing some test shots first to determine the best overall exposure for a single image, then work back from there.