An important question to consider before going down the road of learning to mix ambient and flash: Why do you want to mix constant light and flash in the same scene?
Loads of valid reasons, and it is a powerful photograph tool if you wish to take the time to learn how it works, but working with all the lighting coming from one or the other source has fewer things that can go wrong. So if you can achieve your target end goal without having to mix lighting, then I would strongly suggest taking that route for a project, especially if you do not have a flash meter.
However you can still get very accurate results if you employ guide numbers for the flash even without using a flash meter. They get 'a little tricky' when you add in things like light modifiers or bouncing the light around a room, but it is doable even with complex setups.
Also remember that flash meters make it easier to work with confidence, but a meter alone does not replace taking the time to understand how light travels through a scene and reaches the camera.
What specific flash units are you planning to use? (Suggestions and guidance for using flash cubes and a large format camera will probably not be all that useful to you if you're using something like a fairly new Canon/Nikon system with their TTL flash metering system after all. So the more that people know about your plan the more help they can offer)
But working with flash and ambient light is effectively playing around with double exposure, except the flash exposure does not rely on shutter speed, but instead uses the flash duration for the time component. - Depending on your gear you may also need to watch out for shutter sync speed issues.
A bit of a side note: Learning to work with flash is one of the things that I find a tethered digital system to work wonders for if you have access to the equipment. Doesn't need to be a good camera, and the photos don't need to be anything worth keeping, but being able to make adjustments to the setup and instantly see the results on a decent sized screen can really accelerate the learning process. Once you have a solid understanding of how the lighting is working, and what effect your adjustments can have, then you can approach working with film and flash with far more confidence, and you can skip over the feeling of uncertainty in what is going on.
- Also remember that reciprocity failure might kick in if your flashes can get into a low enough duration. Big grain of salt for lessons learned off a digital camera before switching back over to film, but this is mostly a matter of knowing your flash gear and the film you're using.
