'Africa' constitutes 54 states and literal thousands of ethnolinguistic groups. Some are in dire straits, others (i.e. Namibia, Ghana) are doing reasonably well. Among the trouble spots, circumstances vary considerably - the DRC's predicament is a whole different kettle of historical, political and geographical fish to, say, Niger's. Due to the very nature of European colonial policy, it would be impossible to realize a situation where post-decolonisation Africa is uniformly peaceful without a multitude of little PODs going centuries back.
Agreed. Guaranteeing peace to a massive continent just getting out of European colonialism is pretty much impossible, I think.
There are very few states anywhere on Earth that escaped colonialism/imperialism without falling into extreme violence and/or corruption. Very very few, compared to the states that didn't.
Now since a few parts of Africa are still violent today, there are a few things that could be done now, or could've been done years ago, that could reduce violence somewhat and fulfill the OP's request:
What the West Could Do
1) Have the developed states heavily reduce or end their agricultural subsidies. Most African states are heavily dependent on the export of agricultural goods. The developed world, particularly the U.S., European Union, and Japan, provide their farmers massive ag-subsidies that force down the price of agricultural products. Many African states can't match these subsidies to their own farmers, thus African farmers can't compete, weakening Africa's economy (as a whole) and as we know, economic tensions can cause violence. Somehow getting the West to get rid of these subsidies (pretty much everyone not a farmer recognizes them as harmful to everyone but farmers), would be a big boost to Africa.
2) Promote downstream industries in Africa. Mining is a large part of many African economies. Most companies and states that go into Africa build facilities that get the goods out of the ground, but ship them back home to refine. This limits employment to Africans and takes away income to the states. Something has to shift that makes the placing of refining facilities in Africa desirable.
3) Not support dictators and army coups. Obvious, but I honestly don't see this happening so soon after decolonization. Even if the Cold War were butterflied away, the European powers would probably meddle in the area. To this day France treats its former colonies as a playground.
What Africa Could Do
Africa's in a tough spot at the beginning of independence because it's tied so heavily to European markets and does so little trading in its own continent. It's also by far the least industrialized continent and the least "vital" in terms of the Cold War, meaning that while the U.S and U.S.S.R are very willing to support African states, they won't get the same financial and industrial support as say, the Warsaw Pact members, or Japan and South Korea. Still, there are a few things that could be done.
1)
Stop the Prestige Projects. This is, I think, the most important. At independence, most African states wanted to pull a Meiji (or a Stalin) and rush into industrialization and great-power status. Ghana's Akosombo Dam, Egypt's Aswan Dam, massive spending in weaponry and highways (when most people couldn't hope to get cars), things like this pushed many states deep into debt for no gain other than prestige. Somehow, you need leaders to take a slower pace in the development of their countries.
2) Support agriculture. Many African nations at the outset were either indifferent to or actively hostile to agriculture and farmers, instead favoring industrialization. Considering that most African economies at independence were heavily agricultural, this was an awful policy that ruined agriculture for the kind of projects in my first point. Getting leaders to see the value in promoting agriculture (again, see point one on taking things slow) would do wonders.
There are many many other things Africa needed to avoid excessive violence, but here's a few. Admittedly this is heavily biased to economic reasons (there are a ton of non-economic ones), but these are the ones I think are pretty important and fairly doable with the right PoDs. I think the most important one within Africa's control was to take things slow and steady instead of trying to to a Soviet Five-Year Plan. The excessive debt and economic turmoil brought on by the "prestige projects" and illogical rushes were a big factor in the dislocations and wars of the 70's and 80's.