This is an interesting subject, and it depends on who you are. Generally if you're a snowball champion or you know you can reliably 1v2, getting Rift Herald and maintaining pressure top is often the best bet in lower elos. Also, at this stage you can counter jungle and keep vision on the top quarter of the map making things harder for the enemy jungler, and easier for your own team. (Vision = knowledge = power)
Champions like
are all good examples of champions who get ahead can draw more than 1 person top making the game easier for your team.
You could watch NEACE's art of split pushing, which he goes into depth the actual mechanics of split pushing itself.
Often with some champions your best bet is to float around the area, ward the quadrant near the tower you're looking to push in, so if this is secondary top it'll probably be the Krugs or Gromp. Keep this area warded and keep hard pushing the lane into the tower, but don't commit to any 1v1's unless you confirm where the enemy team is. If you count 4 heads mid, it might be a good time to engage if you can get a hold of the enemy. Watch for people spltting off from the group. At this point you should look to run back and avoid a 1v2 if you're un sure, but if you are sure you 100% should kill them both. This is often achievable with more snowbally top laners, esp Darius, since getting 1 kill often sets up the other kill. Also if you're in voice comms with your team, tell them to force an engage once the 4th enemy team member splits off to come and get you.
You don't necessarily have to be at secondary tower each time to generate pressure, often you just have to let them know "Im near it, and im ready to fucking take it."
If you have a mid lane that is falling behind however, it could be wise to rotate down there after pushing the lane out to help allieviate pressure, but depending on the champ, catching even immobile mages could be risky.
-Sincerely, a Silver Scrub.