The most common tension system on Kenmore 158 and 148 series machines rarely requires adjustment. If you do have to set the tension or if it came apart here are some tips.
Once removed you will be left with the tension discs and adjustment nut. Now you can adjust the tension to what you want before replacing the outer dial parts. Before adjusting the top tension be sure the bobbin tension is not too tight. When holding the bobbin case with a full bobbin inserted you should be able to pull the thread with a bit of tension on it.
Put the parts together as shown and reinsert the outed dial at 4.5 once you are happy with the tension adjustment. Then you can carefully add the other parts and tighten the center screw. Do not over tighten the screw. The “0” and the point of the stop washer should be at the top.