How to use manual flash on a dslr?

I want to buy one of those flashes that hook to the top of a DSLR. (i have a nikon d3200)
I'm saving money and buying a cheap one that is manual only, no TTL. I've never used a flash before, not even the one built in the camera.

i want to know how you know what brightness to use on the flash, and what aperture, iso, and shutter speed to use to correctly expose a picture.

and I'm not doing studio portraits. I need more light for things like taking a picture at a birthday party or something. I'm going to be in situations where i can't take 5 photos and mess with the flash in-between and change settings with each shot. One, two photos, with hardly any time wasted is what i need, but i also need to expose correctly with a manual flash and i don't know how to do it.

the flash i want to buy is called "DBK DF-400 Flash Speed light". Its on Amazon.

so how do you know how to get correct exposure with this flash?

i tried googling it but all the tutorials on google and youtube are pretty bad. And i need things explained very simple.

Ok - try this for starters. You need to have the camera in manual exposure mode (M).

Set the ISO to 100-200 (make sure that Auto ISO is switched off - it's often on by default).

Set the shutter speed at or below the max sync speed (usually around 1/200 sec). As long as you stay below this, you shouldn't get any 'banding' across the frame). The slower the shutter speed, the more ambient light is in the scene.

Set the aperture to f8.

Set the flash power manually to 1/8 power.

Take a test shot. If it's too bright, then stop down the aperture a bit (ie larger f number, like f11) OR turn the flash down by a stop (ie 1/16th power). If it's too dark, do the opposite of the above.

Honestly with my experience it is simply just playing around with the settings. Usually in the room where the birthday party is before the party begins would be a good way. Set the scene just like it will be during the party, and then do some test shots. I try to adjust the settings and just use no flash. Depending how dark it is a flash really isn't necessary and hard to get right. You can usually adjust the aperture and the exposure to get a decent photo without a flash (again depending on how dark it is).

Use Manual mode, set your synch speed (consult your manual) and set your aperture according to flash-to-subject distance.

This worked more easily on MF cameras, where a distance scale gave the range when focus was correct, but the principle is still feasible.

The shutter speeds for using flash are on page 50 of your manual.

I suggest you go ahead and try the built-in flash. Set it to fill flash, like it explains on page 47 of your manual and see if you don't get the results you want.

http://keerok-photography.blogspot.com/2012/08/how-to-use-generic-flash.html

Set your camera to full manual (M) mode
Select 1/200th shutter speed
Select ISO 200
Determine the flash guns guide number at ISO200 from tables supplied with it
Measure the distance to the subject
Divide the guide number by the distance to get the aperture
Set the aperture and fire!

"so how do you know how to get correct exposure with this flash?"
You consult tables, take measurements, and make calculations. You might have difficulty doing such tasks during a party, which is why I wouldn't recommend getting a non-TTL flash. At a minimum, you should be considering the SB-600.

If you can't afford a good flash head, you would be much better served by the built-in flash, perhaps with a diffuser. Even a clean white cupcake cup can help soften the rather harsh lighting of a pop-top flash.

Using Program mode and managing only exposure compensation (+/-) is perfectly fine for situations where you don't have much control over your scene. While you might not have time for choosing exposure settings, you can do quite a bit of post-processing if you shoot in Raw mode and pick up a copy of Adobe Lightroom or Apple Aperture. Otherwise, you would have to commandeer a specific area exclusively for portraits.

Regardless of your lighting choices, remember to keep plenty of distance from your subject (at least a body length) so you do not overexpose faces and underexpose the rest of the scene.