Play level 3, "Grow". The plant is directly in the path of your spirits. Select the first spirit, choose the grow ability, then wait for the second spirit to pass by the plant and have it grow.
Easy "Alternatives" achievement
Play level 14, "Two Sides". You only need to save one spirit. Ignore the spirits on the left. Wait a few seconds until the spawn locations the right become active. When the first spirit from the right-most spawn point reaches the edge of the platform, select it and use the blow power, and aim it to the left. This will force the other two spirits to glide past the flower and into the exit spiral.
Easy "First In, First Out" achievement
Play level 22, "All For One". You only need to save one spirit. Select the bottom right spirit and have it blow directly upwards. Next, have the spirit above it dig down when it is directly on top. This will open a passage for the spirit you are going to save to fly directly to the top of the level. Select the top left spirit and have it blow up and to the right. This will help guide the spirit you want to save away from the spikes and into the exit.
Easy "No One Left Behind" achievement
Play level 29, "A Tree". You must save ten spirits to complete the level while making sure no spirits are killed by the spikes on either side. Grow a tree to reach the exit. Pause game play when issuing commands to make sure none of the spirits pass the one you are using, or they will get blown into the spikes. Ignore the lone plant in the middle of the level. Start growing upwards and to the right as soon as possible. Keep building the ramp until you get close to the right wall and the wind blows to the left. Then, change the direction you are growing and build upwards and to the left. As your spirits jump off the ramp to the right, the wind will blow them back onto the new ramp. You should pass over the small platform with the plant on it and under the exit spiral. When you are just below the spiral and slightly to the left, the wind should be blowing towards the right. At this point, build a final ramp up and right to allow your spirits to escape.
Easy "The Apprentice" achievement
You will need a cumulative total of thirty plants from any number of completed levels. Because your best "plant score" for each level is used, you cannot complete the same level multiple times. "The Apprentice" will be earned after you complete the level on which you collect your thirtieth plant. Your progress can be viewed at the bottom of the level selection screen.
Easy "The Other Way" achievement
Play level 18, "Cave Story". You only need to save fifteen spirits. You must complete the level without using spirits from the spawn point found at the top left of the map. Use two spirits from the middle spawn point to build a ramp up towards the exit point, then use a third spirit to dig to the left in order to open a path to the exit and create a small platform. Wait for a spirit to get onto that platform then blow straight up. The positioning can be slightly difficult because it must be just to the right of the left end of the platform. Next, make a tunnel for the spirits on the right to reach the exit. Wait for a spirit to walk as far left as possible, then use dig towards the left. Repeat this with a second spirit. Wait for the spirits to hit the exit spiral to earn "The Other Way".
Easy "A Plant" achievement...