For beginners, you may not know what a backstitch is. It's elementry in sewing, and you need to know it. Backstitching is essential when sewing by hand or on the machine.
What is it exactly? This secures the starting and finishing seam that was just sewn. The seam will without fail unravel if you don't backstitch.

Each modern machine has a backstitch button. Even the majority of older machines had them. No matter which one that is used, the process is the same.
Typically the button is just a symbol of an arrow that points backwards. We will use curtains as an example.
When you start and stop a seam, the backstitch will need to be used.
First sew the first 5-6 forward stitches as you normally would. If you have a manual button to backstitch, and you are new to using it, you may want to slow the machine down a bit.
Just until you get used to it. After you have sewn the first 5-6 forward stitches, you then can push the backstitch button for the backstitch.
Many of the modern machines will auto backstitch without holding the button down. But with some, you will have to hold it down the button while it backstitches.
5-6 stitches is plenty of backstitches for the beginning and ending seams. The fabric feeds backwards as well while this takes place, so keep that in mind.
When backstitching, do your best to guide the thread over the stitches that have just been sewn. If it's not exact, don't worry. Just get it as close as possible.
Once you have backstitched the 5-6 stitches, start to sew forward again. If you have a manual backstitch button, then you will have to hold and release the button until you are finished with the backstitch.
As mentioned previously, be sure to backstitch at the end of the seam as well. So the process goes this way.
First, sew a forward seam 5-6 stitches. Next, backstitch. Follow this up by sewing all the way to the end of the seam, and then backstitch once again for the final stitching of that seam.
Now you’re done, with a nice secure seam. This prevents the thread from unraveling at the beginning and end of the stitching.
To get a good visual of what a backstitch looks like, be sure to watch the video below: