We started our trek from Tarsar lake to Sundarsar lake at 7:30am in the morning.
The only part you might be scared of is the tarsar pass but in reality it's easy it's not too hard and once you cross that after that it's a beautiful walk through a meadow till Sundarsar lake.
We went to Marsar lake on the same day to watch sunset there and then came back to Sundarsar lake and that sunset was so incredible and so calming. No one wanted to go back but we had to.
P.s You can't camp at Marsar lake.