There are four different locations in the game: The Classroom, The Backyard, Alex's Bedroom, and The TreeHouse, containing 16, 32, 32, and 32 levels respectively, for a total of 112 levels. There is an achievement for completing each of the worlds, as well as an achievement for earning 3 stars on all the levels of each world.
Completing a level simply means satisfying the objective, and nothing more. When you first load up the level, pay close attention to the blue lines that are drawn on-screen. Those lines are the only way to know the objective as it is not explicitly spelled out for you anywhere. Objectives include getting objects into baskets, knocking things onto the floor or through trap doors, popping balloons, getting a toy car from one side of the level to the other, having balloons fly up and off screen, breaking a piggy bank, etc. If you forget what the objective was, just refrain from touching the screen for about 10 seconds and the lines will be drawn again for you. Completing levels within a given location unlocks more levels in that location.
Earning 3 stars is much more challenging than just completing the objective. Not only do you need to complete the level, but you also need to collect the 3 yellow stars scattered around the level while doing so. Stars are collected when any object in the scene hits the star. It can be an object you placed, or one that was initially on scene and unmovable. You will need to consider these stars when you are allocating your given resources. You also need to make sure you collect all of the 3 stars in roughly the same amount of the time as you complete the objective. This is important because completing the objective ends the level, meaning even if you are about to collect a star the level will just terminate (you get about 1-2 seconds after the objective is completed).
Because of the need to collect all of the stars while completing the objective, this game can be quite challenging and time consuming to undertake without using guides. For the quickest completion, it is suggested you use guides for every level. Below are links to video and picture guides. Picture guides are recommended over the videos, as it takes less time to look at a picture than it does to watch the video for each level. However, in some cases, it is beneficial to actually see how the level is supposed to play out. This is when the videos become useful. Also, some solutions to the levels need to have very precise placement and you may not be able to get it to work. In this case, switching to another solution for the same level is a good idea, as the other one may be easier to replicate.
LINKS TO PICTURE GUIDES:
All-in-One Guide - 90% of the levels
The Classroom - ALL LEVELS
Amazing Alex - All Levels Classroom - YouTube
The Backyard - ALL LEVELS
All Levels backyard - YouTube
Alex's Bedroom - ALL LEVELS
Amazing Alex Bedroom All 32 levels - YouTube
The TreeHouse - ALL LEVELS
Amazing Alex The Treehouse All Levels - YouTube
Video/Picture Guide Credits: Markham Asylum at DeltaAttack.com, Mark Brown at PocketGamerUK, AmazingAlexWalkthroughs.com, absurb on InsideGames.com and YouTube users HanjoHoubein, 3stargoldenegg, and MrSinghOfficial.