It seems that i3 is more difficult to master than other desktop environment Fedora ships. I will assume that what you downloaded is Fedora-i3-Live-x86_64-34-1.2.iso and you have already entered the live environment. If you haven’t do that, follow the instructions on Fedora Docs.
Now answer the questions shown on the screen according to your preferences. The following choices are based on my personal preferences, please notice that the key combination later used may be different if you don’t follow my path.
Step 1. I pressed Enter key here to generate a config.
Step 2. I chose the Win key as the default modifier so I pressed the Enter key directly here.
If you want to choose the
Altkey as your default modifier, press the down key first then pressEnter.
Step 3. Now open the terminal with the Win + Enter key combination.
This is documented in the i3 user guide and in the user guide the
$modkey is what you select in step 2, so if you choseAltin step 2, then you should use theAlt+Enterkey combination instead, the same below.
Step 4. Follow what the terminal says, that is, use the command liveinst to launch the installer.
Step 5. The installer will appear right next to the terminal so you may wish to move the terminal to another workspace and only keep the installer in workspace 1. Move your mouse cursor to the left half of your screen to make sure the terminal is activated, then use Win + Shift + 2 key combination to move the terminal to workspace 2.
This is also documented in the i3 user guide. You can switch between workspace with the buttons located at the bottom-left corner of your screen.
Step 6. Follow the instructions on Fedora Docs to finish the installation.
In fact, if you already know that liveinst opens the Fedora installer, there is another method which requires less operations. But I didn’t know about the command until I opened the terminal. 
Step 1. Same as what I mentioned above.
Step 2. Same as what I mentioned above.
Step 3. Open dmenu with Win + D key combination, then type liveinst, then press enter.
This is also documented in the i3 user guide, dmenu will appear at the top of your screen.
Step 4. Follow the instructions on Fedora Docs to finish the installation.