EDIT: I just saw the second response from Altheabyanothername about using boiling water - I didn't know how to respond to that mention as well, so I decided I'd edit my original follow up message about that.
The boiling water idea sounds fantastic! I will definitely do that! I have a quick kettle boiler (I drink a lot of tea!) so that will work out perfect! Thank you so much, and I'll be sure to let everyone know how it turns out!
(original follow up comment)
Thanks for your response and your advice!
The first couple options of cooking soil in the oven and whatnot, admittedly, sound like a lot of work and kind of complicated (and beyond my abilities!)
I'm not against using weed killer whatsoever, in fact, I'd prefer to use some kind of spray on the soil, before planting the flowers later this week. (I should also add that the flowers I buy are...already grown, for lack of better terms? I get them when they are about 6 to 8 inches tall, so I'm not planting seeds or anything of that sort.)
My concern was, would using a weed killer on the soil ahead of time before planting the flowers, end up killing the flowers too? I know nothing about how that works. From my total novice standpoint, I just imagined that those power weed killers would kill everything and anything it comes in contact with, and would affect the soil which would in turn end up killing the flowers.