The Nikon FE doesn't have TTL flash - the FE2 has it. Using a flash to provide side lighting will be a little tricky without a light meter. Luckily a decent flash light meter is fairly inexpensive. I bought my used Sekonic L-328 for ... eh ... maybe around S$120? The L-328 supports the use of a 5-degree finder for reflected readings as well.
If you were to use the flash's built-in auto control, you may find the result a little unpredictable. This is because the flash's sensor assumed a direct flash setup. For a flash without built-in auto control, you can use the guide number and use that to determine the f-stop setting. You can read about guide number here:
Flash Primer 3 - Understanding Guide Numbers
But again, if you are firing the flash from the side, even with the guide number, you'll still have very little control over the lighting ratio from both sides of the subject without a meter.
So, get a flash meter
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