Wash 7 half-pint jars, lids, and bands with warm soapy water and rinse thoroughly.
Place the jars in a water bath pot (basically a very large stock pot) on the rack, fill with water to 1 inch over the tops of the jars, and heat over high heat until it comes to a boil.
Boil for 5 minutes, then add the lids. Boil for an additional 5 minutes, then turn off the heat, removing the jars and lids to a towel to dry.
While you are sanitizing the jars and lids, prepare your strawberries. Place in the bottom of a large stockpot (you'll want something with high sides so you don't get splattered by hot fruit) and mash to the desired consistency. You can also use a stick blender (or regular blender) if you'd like a finer consistency.
Stir in the pectin and heat over medium to medium-high heat. Bring to a boil.
Add the sugar all at once and stir until it is dissolved.
Add the vinegar and keep heating until it reaches a boil again, then keep it on the burner for another couple of minutes, stirring the whole time.
(You can test the consistency here by using a cold spoon and scooping out a little bit onto a cold plate to see how set the jam is; I prefer mine a little looser, but if it's not set enough, you can cook it longer until it is. Just retest periodically til you find it's set the way you'd like.)
Remove from heat and scoop off the foam, discarding it (I ate it. I couldn't help it; I couldn't waste it!).
(Turn the water bath pot back on high heat to be sure it's boiling when you need it.)
Place the funnel into the neck of the first jar and start ladling the jam into it. Fill, leaving 1/2" head space. Knock the jar on the counter a few times to be sure the air bubbles are worked out. Wipe the rim with a clean, moistened cloth.
Using a lid lifter, gently place a clean and sterilized lid on the top of the jar. Add the band and hand-tighten (you don't want to over-tighten).
Repeat until all the jars are full and covered.
Return the jars to the water bath pot and process as recommended for your altitude. 10 minutes is a safe bet, though technically at sea level you need to only do it for 5 minutes.
Remove jars to a clean towel and leave undisturbed for 12-24 hours. You should hear the lids ping some time after this. If, after the 12-24 hours, any of the lids have any give when gently pushed on, place the jar in the fridge to be used within the next 2 weeks. Any properly sealed jars can be stored at room temperature up to one year.
Use common sense when it comes to canned goods. If, when you open it down the road, the jam doesn't look or smell right, discard. Do not taste test it first!