This guide assumes you are playing with Victoria Revolutions and know the basics of the gameplay.
Transvaal may seem like at an extreme disadvantage at first. Landlocked from the coastline by the remainders of Zulu empire, meagre profits, no army to speak of except for the reserves and a potentially devastating war with England on the horizon. Playing your cards right is then a priority and achieving great power status though difficult, becomes quite possible if you know what you're doing.
To begin with, annexing Oranje and later on Zulu should be the priority. I suggest declaring war on Oranje as soon as you see the welcome screen and mobilizing your 3 divisions, while throwing your only infantry into battle. Oranje will not resist much and you'll be able to annex them by 1837. The problem will be getting out of the debt you'll definitely get into and that will take a few years at least. This is not going to be a problem since by 1840 you'll have discovered diamond deposits in Transvaal and this will provide enough money for the time being. It's probably best if you demobilize and convert all pops at the precious metal province to labourers so they all increase your profits to the max. You'll instantly see a huge boost in your profits and meanwhile should try to research industry, economic and military techs.
These twenty years should be spent in development and research. Somewhere in between when you can afford to make two divisions, use them to annex Zulu if you haven't done so already and I also suggest annexing Madagascar for those extra divisions. Your main goal however is not military pursuits at the moment but trying to create the most liberal society on the planet. Yes, you heard me right, we're going the banana republic way and will try to attract as many immigrants as possible because our population is miniscule on its own. So as soon as you have enough profit (6+ pounds per day), start slowly increasing the social reforms (you might need to switch your party to interventionist or state capitalist one from the default laissez faire) and proceed to increasing all reforms to at least trinkets or preferably low levels. It'll decrease your profit, but as long as you're in the positive, you should be fine. When that's done, switch your party to liberal (assuming you're still presidential dictatorship like in the beginning) and change political reforms to suffrage, public meetings and free parties. This will give you democracy and a liberal party in power. With this and social reforms (as well as a port from Zulu empire) you should start seeing an influx of immigrants. Probably Irish, French and Dutch at first, but more later. Produce one capitalist POP somewhere in Transoranjia if you still haven't and decrease capitalist taxation so that they start making factories. With factories, you'll start seeing craftsman immigrants, which will fill your factories for free. n From now on, all your factories will be filled by immigrants for as long as you keep the social reforms in place (you can reduce social spending to 0, it won't change anything, but will make your POPs get conscious very quick), have a liberal party in power, don't have any debt and have a democracy. Plurality will also help, but you can't do much about it anyway, just keep liberals in power and you'll see it rise after some later events. If you suddenly stop seeing immigrant influx, check if you're in debt, it's usually the first thing that stops them from coming.
There are a few things you can do now, either go full defence and dig in along the entire Transoranjia border, relying mostly on immigrant influx and wait for the UK to declare war on you. Using trench warfare, you can successfully hold your positions for years and they'll probably ask for white peace at some point. You'll definitely need fortifications on all sides for that and especially in the flatlands or hilly areas. Mountains are usually much harder to take and AI won't even try it.Since manpower is your main problem, try building guard divisions instead of just infantry, that way you'll save up on manpower and have high quality troops defending your borded. If you choose to go full defence, by 1899 when (or even if) Boer war begins, you should have 20+ million population and a huge profit, assuming you've had immigrants for at least most of the time. Whether you pursue colonization of the rest of Africa is up to you, but doing so will negate all those defencive structures you built before claiming new colonies.
You can also choose a more active approach, i.e. go with the saying "best defence is good offence". This of course means planning an attack on the Brittish, while also actively pursuing colonization to the north. The best time to attack Brittain is somewhere before 1870 when you have enough forces to quickly take and hold South Africa, where they usually keep minimal reinforcement and also do so while Brittain is at war elsewhere. One of the multiple wars with Russia would guarantee that all Brittish forces will be off the African mainland and you can use that opportunity to strike quick and strike hard. You'll need at least 12 divsions to take and hold the coastal areas of South Africa, while also making sure you have enough manpower to constantly reinforce the forces while they're in the enemy territory. If you manage to take their South African holdings, holding the mountaneous costal regions should not be a problem. If however they managed to bring reinforcement, don't be shy to retreat to your fortificated homeland and wait out until you are ready to strike again after the peace treaty. It might be a while before England agrees to cede the entire South Africa to you, but as long as they offer you all the coastal regions, take it, since they won't be able to reinforce the lands in the middle of another country.
Finally, another alternative is a bit less interesting, but can give you an upper hand in colonization of the rest of southern Africa. You can simply purchase the two northmost provinces from the Brittain that border the uncolonized land and then devote your entire attention into colonization. You might also want to get an alliance with the UK going since they will not declare on you then and slowly buy off the rest of the African domain you're interested in.
Remember, that depending on the path you choose, you should plan in advance your research. If you go full defence, spend extra time researching military so you can get fortifications earlier on. If you want to colonize, you need to make a compromise with few things and make sure you get medicine, machine guns or imperialism as soon as you can. If you are going to fight the Brits, you'll probably want to have a lot of industry and military and if you want to buy the Brits, you'll want industry and economy.
Having survived, defeated or bought the Brittish, your main concern will be trying to get as many points as you can since there won't be any major enemies unless you make some. If you choose to go for points, try changing your party to state capitalist or planned economy with full citizenship and build tons of factories in the newly colonized African domain. Since it is your main continent, you can build factories anywhere you please as long as it's in Africa and that means you can quite easily get the highest industrial score and be in top 5 Great Powers by the end of the game. If you begin colonizing further on and impede on the French domain, they might declare on you, or you can even do so yourself and take their lands piece by piece, increasing your overall population and building factories in the newly acquired lands. If your goai is unification of Africa (which I am yet to achieve since 100 years is just not enough), you are probably going to stay a lower great power (#7-8), but it might give you a sense of personal achievement or something.