Simmer the strawberries and agave over medium heat until the berries are soft. You don't want to boil that could damage the flavor, so just simmer.
Then puree the berries, adding the lemon juice if you like. You can use a food processor, but I find that my immersion blender does the job well and it's less cleaning to do.
Let the mixture cool in the fridge, before adding the yogurt.
Put into your mold and freeze until solid. The time it takes depends on the size and shape of the mold.
Notes
The amount of pops this will make just depends on the size of your molds. I ended up with 4.