The warpless route is very straightforward, it just involves a lot of platforming. The first swinging monkey group in the final area can be short-hopped under. The last one can be simply ducked under, or jump into while simultaneously tonguing backwards. If you do this second option, the monkeys will grab baby mario, but the back tongue will automatically retrieve him.
Any% w/ Warp
The warp in 3-1 is triggered by falling above the screen transition area in the trees. Normally, to get to this area, you would take a path down the mountain backwards through the water, and hit a spring arrow that bounces you into a bonus room. If you instead bounce on a monkey and flutter on top of this area, you will trigger the 1-1 warp. It can be tricky getting the monkeys to cooperate, as their movement is highly unpredictable.
There aren't really any tricky or difficult sections of this level, just a lot of small things. Beware getting hit in the 2nd half of the level, as there are no good sources of backup stars.