This is the video that we went off of. We used almost the same materials the person in this guide used. If the video helps you better understand how to make a portable retropie then feel free to watch it instead. In the end you should have something that should look like this.
Step 1: Tools and Materials
We used our own portable charger and if you want you can too. Just make sure its small enough and doesn't weigh too much. http://amzn.to/2CiZcO0
3.5 inch screen: https://amzn.to/2L8wy6l
Raspberry pi3: https://www.amazon.com/s?k=raspberry+pi3&ref=nb_s...
Games g3s controller: http://amzn.to/2BqtaTn
You should be able to use whatever controller you would like. The video uses controllers with a stand on it but it shouldn't matter.
Buy velcro or tape
Step 2: The Building Steps
First you will need to put the raspberry pi and screen together. At the end of this it should all look like this.
Step 3: First, Follow Instructions on Screens Box.
When you buy the 3.5 inch screen it should come with an HDMI dongle, the screen itself, and a case for the screen and raspberry pi. On the box with the case in it, there are instructions to put the case together.
Step 4: Next
Apply some sort of velcro or doubled sided tape to both the raspberry pi and your portable charger. We used velcro.
Step 5: Retropie
If you haven't already, you will need to download retropie onto your rasberry pi's SD card.
Step 6: Final Project
Now all you should need to do is boot up your raspberry pi and plug in a controller.