This is a level you’ll have to play more than once but the first time around it’s best to just grab all the Green Stars then grab the Stamp on your second trip. Start by hopping on Plessie and a little ways down use the Splounders to bounce up to Green Star #1.
From here stick to the right side and bounce up to the Red Ring and collect all eight Red Coins along the upper path for a 1-Up. Stick to the left side and bounce up the tiered water levels to reach a 1-Up Mushroom then dash through the waterfall on the left side. Claim Green Star #2 here and a Super Star which will help you move through the final area.
Use the Dash Panels to get some speed and jump off dead center of the final platform to reach Green Star #3 and plenty of Coin Rings.
In a replay of this world take the right path at the waterfall. It winds left and right but the Stamp is found here.