What I would say from my experience is (my LL is about 5 seconds, f2l about 10) , the follows
Pick up good algs, drill them individually, drill them so much. And set landmarks for them. Like first aim to get all your PLLs(cuz they are less) sub3. Do same for OLLs. Then try to get all of them sub 2.5. Then sub2. With the standard algs and not so good fingertricking, sub2 is certainly not that impossible on any alg. Same for OLL. This will make your LL about 5 seconds. After that, if you see how great people do it, and perfect your finger tricks, (adapting to some really weird ones), you can sub4 or even sub3 the LL. But 5 seconds is something that just takes spending time with the algs individually, and timing them(Say like an average of 100 with each). I did this a lot, and my LL is quite decent, though fingertricks are very basic, and lookahead sucks in last layer, but 5 seconds it is, TPS is quite decent
(If some specific cases, like 4 or 5 , trouble you, and you just cant get below the landmark you want to achieve, then message me, or post on this forum, there will always be advice for you

)