This comp uses the Yordle trait and slow rolls for Poppy and Tristana as the main carries. Poppy is the core carry of the comp as she will tank extremely well. Tristana is the main backline carry, and you should prioritize slow rolling for her at level 6. The Yordle trait will give you more Yordles to 3 star Poppy, so level 6 slow rolling is ideal.
Go for this comp if you have a good start with Yordles and defensive items for Poppy.
In the early game, don't level in order to save up gold.
At Round 2, level up to 6 regardless of your start. Put together the strongest team you can. You can roll a little bit here to find at least Poppy 2 star and Tristana 2 star if you don't have them already.
Once you hit 50 gold, start slow rolling for Tristana and Poppy 3. Once you find them, you can begin catching up in levels either slowly (with gold above 50), or you can spend most of your gold to hit level 7 to aggressively keep up with the lobby. Reach level 8 once you can afford to and finish the team comp.
Simple enough, run the Yordle units along with supporting units that you find.
At this point, you can run all 6 Yordles for the bonus to all Yordles, reducing thier mana cost. If you haven't found all 6 yet, you can continue to run other supporting units.
Some games, going for more 3 stars other than Poppy and Tristana can be a trap. More three stars from the Yordle trait is great, but leveling up to add in more utility units is also important.