(S) To get to the start of Zelene Pleso, you need to get to Tatranska Lomnica. From here you can either take a cable car to Skalnate Pleso or hike up the ski hill. The cable car is not free, but it allows you to skip a couple of hours of a very boring climb up to Skalnate Pleso.
Also, note that Slovakia is known for some of the best trail markings around the world, so it's very easy to follow any trail in the High Tatras. There will be a map legend at almost every trailhead, make sure to check the destination or a highlight you want to go to and memorize the color of the trail leading there. You will soon notice that every other tree along your path will have colored markings that show you're still on the right trail. At a fork, you will clearly see different color markings, so you'll be able to know which of the paths is yours.
(1) When you arrive at Skalnate Pleso, follow the red trail (North-North-West) name Tatranska magistrala) to Velka Svištovka Mountain (2037). The trail ascends pretty gradually and has no difficult sections all the way to the mountain pass.
(2) At the pass, you can choose to climb the Velka Svistovka Mountain (North-East), which is easily reachable from the pass, but you will have to ascend a little higher for that.
After the pass, the trail goes down fairly quickly. There will be numerous switchbacks leading you down the mountain towards the Zelene Pleso lake. You will be able to see the beautiful green lake from your switchbacks. This section is slightly more difficult, but very manageable if you don't rush too much. The switchbacks can become boring, but the trail is pretty steep, so careful not to slip and fall.
(3) Before the end of the switchbacks, there will be a section with a chain you have to follow. It's not exposed in any dangerous way, simply the trail section becomes quite uncomfortable and steep to descend so the chain offers you some balance.
After that, just follow the clear path to the lake and the mountain chalet at Zelené Pleso lake. Here, take some amazing images and a few refreshments.
(4) To head back, head east, towards a clear gravel path that descends into the forest. There are numerous ways you can head back to Tatranska Lomnica, however, this guide describes one that has more shade and gives you a couple of more opportunities to enjoy the diverse landscape of the High Tatras. For that, this guide follows the Yellow-Blue-Yellow-Green trail markings. You will start your gradual descent by following the yellow markings eastwards.
(5) The yellow trail then will turn south-east at the creek, keep following it.
(6) You will be able to see another trail on the other side of the creek along your route. That is the blue trail. Once the yellow trail merges with the blue one, keep following the blue southwards.
(7) Later, at the fork, the yellow trail veers off to the left and the blue one leads to your right. Continue on the blue trail. The yellow trail at this stage would lead you to Bela Voda car park, so unless you organized your trip to end there, make sure to stay on the blue path at this point.
(8) Keep following the blue trail which is leading you south, towards Tatranska Lomnica. The trail itself is very easy at this stage and you will soon start seeing the first buildings of the town. You will then see another fork of the trails splitting. At this stage, take the yellow trail, which keeps you on the village paths.
(9) At this stage, you're already in Tatranska Lomnica, but there will be more trail markings you will spot. I recommend following the green trail as it will lead you directly to the town center, where you can easily get the electric train from, provided this is the method you used to get here. (E)