Use the brown bricks to form footbridges for R2 and go through the pass. Destroying one of the bushes near the shore of the puddle will reveal a grapple point that you can use. Notice that R2 can just swim across the swamp. Switch to the little guy and float to the platform to the left. Use the panel there. When Luke joins you, blast through the bushes to the next section.
Fix a button with some loose bricks and press it. Use R2 to float to the platform that appeared. Continue pushing the buttons with R2 until you're on the other side of the lake. A cut-scene will occur now.
Approach Yoda and push the character switch button to have Luke carry him piggyback. Now you can use the Force. Tear out all the flowers around and use the lightsaber to destroy the flowerbeds. Build the bridge of what's left of them. Move to the central isle and jump on the button sticking out of the swamp. Use the next buttons to reach the other side.
Use the Force (if you've lost Yoda, you'll have to get him by returning to the central isle) to lower the two "mushrooms" and then immediately switch to R2 and enter the lower one. When it raises, float to the other one. You'll have to be quick here. Use the panel that you reached and go through the door.
From now on Luke can use the Force on his own, and Yoda is playable just as any other character (and is still kicking but just like he did in the previous Lego Star Wars). It's worth noticing that if you go down the screen from here, you'll be back to Yoda's shack - it's good to pay a visit there to get the studs, especially if you're running for True Jedi. Use the Force on the bricks and you'll build a tractor (sic!). You can use its trailer to form a drive for R2. Switch to him and float to the left to reach the panel - use it to allow other characters to cross. Smash through the wooden grid and go forth.
Jump through some platforms across the abyss. At the end you'll have to fight Vader. When you hit him a few times, he'll jump to the left platform. Stand on one of the blue crates in there and use the Force on the other one. Yoda will do the same and you'll both reach the upper level. Cut Vader a few times more and he'll escape again, this time to the right. Use the crates to build stairs for yourself. Now finish the job and go through the newly opened passage.
Use the Force to tear out the flowers and build a turnstile. There are two crates to the right that you can use together with Yoda to elevate yourselves higher. When you're up there, push the crate down and build a motorcycle of its remains. Now pull out the laundry machine (this game is just sick) of the swamp nearby. Put the motorcycle into the laundry machine and use the Force to turn it on. Now build a panel for R2 from the loose bricks that appeared. Use it and you'll see your X-Wing. Switch to Yoda and pull it out of the swamp.