Remember this moment, because it'll be the first of many where you'll have to think ahead with your portal placement. Now all you'll have to do is ride the moving platform back to the orange portal and walk through towards the elevator. Then place your blue portal through the door before it closes, and put your orange portal on the wall you used to get here. Get to the upgraded portal gun as usual and press the button that pops up next to it after you do so. Just pass through the momentum checks with your portals and you'll be able to move on to the next chamber. Since you can't start with Test Chamber 11, you'll have to get through this one real quick. You can see that run below, with extended pauses being cut from the video. I managed to get it in about 50 minutes, and that's with being distracted by food, texting, etc., so it won't take very long for you either. Despite all this, the achievement isn't actually that hard to get. There is a small noise that lays when you pass through a portal, which you can use as an indicator of whether or not you may have accidentally passed back through a blue portal, but it's always just easier to abstain from sitting between a portal for any extended period of time. With the parameters for the achievement as they are, it's a good idea to save often and try to avoid sitting in the middle of a portal for very long. A benefit to you is that this only applies after you have the fully upgraded portal gun, which also means you can start a new game from Test Chamber 10 (11 is when you get the gun) instead of starting from the very beginning. For your second run of the story, you'll be going for Out of the Blue, which requires you to only enter through orange portals and exit from blue ones.

Test chamber 19 portal 1