The hike to First, which is a cable car station located partway up the slope from Grindelwald, is a steep and challenging hike with fantastic views along the trail and at the top. The hike departs from Grindelwald train station, which is quick and easy to reach from Interlaken. For timetables, check sbb.ch. At the train station, find the main large yellow hiking sign and follow it toward First. Keep following the signs toward First and Bort (if there are two pointing in different directions, choose the one with the smaller time estimate). The first 2.4 kilometers are on roads, but that doesn't mean it isn't steep. From the beginning, the hike is sharply uphill. When the road eventually becomes a gravel trail, the hill does not get any easier.
At around 3.7 kilometers there is a house on the left (there is a water fountain here, though drink at your own risk). There is also a hiking sign here, and you can decide which path to take. The straight/left path is slightly shorter to First, but the path on the right passes through the Bort cable car middle station. If you take this path, you will pass Bort at 4.4 kilometers. There are a few restaurants at Bort, and from here you can take the cable car either up the rest of the way to First or back down to Grindelwald.
From Bort the hike leads along a fun but very steep and challenging trail. Head to the northwestern side of the cable car station and follow the road up the hill for about 100 meters. There it crosses the river, and the trail continues to the right up the hill. This section has lots of switchbacks and is a bit narrow in some places. At the end of this section you reach Waldspitz, which has a bus station and a small restaurant. Here you continue to the right (northeast) along the gravel road, only slightly uphill. There are great views of all the surrounding peaks in this section. Soon you will reach Bachläger, where there are lots of waterfalls surrounding the trail, including a few large ones off the cliff to the right. In summer there is an animal trough where you can most likely refill your water.
From here First is within a mile. The trail leads cuts along the slope with only a few switchbacks. This section has amazing views and interesting and fun terrain.
When you reach the top, you will see the sign for the First Cliff Walk. Here you can choose to walk along the suspended bridge and platforms along the cliff, or you can avoid this section to the left and take the hiking path. If you are not afraid of heights, the cliff walk is recommended. It's only about 5 minutes end-to-end, and it is a fun experience. (There is no fee to do the walk.)
At the other end of the paths you will find the cable car, restaurant, and Berghaus/cabins. There is also a long platform (technically part of the cliff walk) that extends several meters out above the open air and gives a better unobstructed view of the surroundings.
Stop and enjoy the views for a while - then you can either take the cable car back down or hike back. If you hike back, you can choose to retrace your steps or take a slightly different path back to Bort. See the official Swiss online map for details. The hike is 17.4 kilometers round trip, or 8.7 kilometers one way.
Congratulations on finishing this steep and difficult hike! While you're at First, don't forget to visit Bachsee (Bachalpsee), which adds only one to two hours round trip depending on speed.