Although it's always best to use fresh berries when possible, wonderful jam can easily be made from frozen strawberries as well. Defrosting your strawberries beforehand will speed the process along, but you can use them straight from the freezer as well---it will just take a little longer to reach jam stage.
Place strawberries, lemon juice, sugar and pectin in the pot. Stir together; if berries are frozen, you may have to wait until they defrost.
Turn burner up to the highest level of heat that it can reach. Continue stirring.
Observe your jam as the strawberries start to turn to mush and the mixture begins to come together. Stir occasionally; stirring continuously is not necessary. If you are comfortable, it is even possible to be doing something else when you've reached this stage in your jam-making. Just make sure you're close enough to your pot that you'll notice when it starts boiling.
Bring jam to a rolling (or hard) boil and leave it there for one full minute. Add butter and stir into jam. After jam has been at a rolling boil for one full minute, turn the heat off.
Funnel jam into prepared home canning jars and can in your usual manner.
It is possible to make strawberry jam without pectin. In fact, that's probably how your great-grandmother used to make it, before pectin was commercially available. However, that process involves reducing a lot of the natural juice out of the strawberries---and also kills their nutritional value. If you wish to try making strawberry jam without pectin, omit all ingredients except the strawberries and the sugar. Use 12 cups of sugar instead of eight, and boil until the jam starts to thicken. Ladle into your prepared home canning jars as above. It is also possible to make strawberry jam without added sugar, for those with dietary concerns. See Resources for a link to a recipe and some tips and tricks on how to make a batch of tasty, sugar-free jam.
Do not underboil the jam, or else the pectin will fail to properly set. Instead of strawberry jam, you'll have strawberry sauce. Do not overboil the jam, or else the pectin will start to break down, which will also result in a failed jam set. One minute is all you need at this batch size with commercial-grade pectin. Do not double the recipe. If you wish to make multiple batches of jam, you'll need to repeat the steps for each batch, unless you are working with commercial equipment. Most home cookware does not heat and retain that heat as evenly as commercial-grade equipment does. Since jam made with pectin is so sensitive, it is imperative that it be heated evenly.