If you aren't using your flash in automatic mode, you're going to need to figure out things like aperture and power levels on your own.
But first you need to know how much power your flash has, and for that you need the guide number.
The guide number is a figure that shows you the flash's power, based on a formual of distance times aperture. The higher the number, the farther away a subject can be and still illuminated adequately.
In the old days, we would have to do some math to figure out the aperture setting for a subject at a particular distance, or use a table on the back of the flash that would tell you what settings to use for a subject at varying distances.
Today, if you want to use your flash manually, there are calculators you can use on your phone, such as the one in the Photographer's Companion, where you input a few variables, such as the guide number, flash power and either aperture or distance, and it will do the rest of the work to find the right camera settings.
If you don't know the flash number, you can look it up in the flash manual, which you can also upload to your phone so you'll have it at all times.
If you have just one flash, remembering the guide number is simple enough. But what if you use more than one flash, and they have different guide numbers.
What I have done is made a label showing the guide number that I affix to the side of each flash unit, so I can tell with a quick glance what the guide number is when figuring either how far away to put the flash, or what aperture I'm going to need to get the right look.
When I made my labels, I used a word processing program and set it for an inverted output, with white numbers on a black background, making the label look inconspicuous, even more than the old tables that used to be on flash units.
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