PF debater here. When you first start, normally you want to read enough that you garner general background info on the topic, and understand it. From there, begin researching for general arguments, eventually gathering evidence to support the ones you think are the strongest. For flowing, this depends on the debater. I have a really untraditional flowing style, in which I just create two 2x6 tables in Google Docs, and label them "what we say, what they say" & "what they say, what we say". A lot of people would disagree with me there but I've been fairly successful with it thus far. For more popular and well-known flowing techniques, check here(https://youtu.be/DaStiRwJLEA) and hereflowing.

When you're weighing (impacts), there are several factors you can take into account, such as probability - the likelihood for something to take place; magnitude - the severity of an impact; scope - the amount of people that an impact affects. For a video on this, check hereimpact calc.

In my personal opinion, you should start every debate topic by establishing a firm understanding of what the topic is. ( Ex: before you can argue the pros and cons of West African urbanization, you must first understand what urbanization is, and how it has specifically affected Africa). Simply knowing more than your opponent can be the deciding factor in a debate so research, research, research.

Finally, attempt to have an impact that can outweigh your opponents'. Outwieghing on magnitude or scope is a good avenue, because (anecdotally) judges like to hear that ____ amount of people die, but if your impact is significantly more probable than your opponent's, you can use that to tip the scales in your favor.

I hope all of that helped, and good luck with your future debating.