At a very basic level, all you need is a text editor to write the game with. There are many options for that.
Here’s how I would do it if I could only use my phone. Instructions for Android.
Download ChoiceScript
Go to ChoiceScript’s github page and download the source code for the engine.
Using a file manager (I recommend File Manager, yes, that’s the name), move the zip file to an appropriate folder inside your mobile instead of leaving it in the download folder.
Install Spck Code Editor
There are a couple good code editor apps out there. I personally recommend Spck Code Editor.
Open the Project
Open the zip with Spck and start editing the files inside web/mygame/scenes. When you want to playtest, open the index.html file inside mygame folder and press play on the top right-hand corner.
You can run them from Spck.
Good luck!