Electra Team announced recently the release of Chimera, which allows jailbreak on iOS 12.1.2 and numerous iOS devices as well as tvOS 12.1.1.
Before proceeding, make sure to back up your device using iCloud or iTunes to avoid any problems;
Step 1: Download the latest beta version of the Chimera jailbreak from here and Cydia Impactor from cydiaimpactor.com.
Step 2: Right-click on Impactor.zip and extract the archive;
Start Impactor.exe from the extracted folder. DO NOT run the file as an administrator;
Step 3: Click on Run if required to confirm;
Step 4: Connect your iPhone to your computer via USB making sure to select Allow if required on the device and Continue if requested by iTunes;
Drag and drop the Chimera IPA that you downloaded earlier in the Cydia Impactor app.
Step 5: Enter your Apple ID and password. This is only used to sign the IPA, in order to run on your device.
Step 6: Cydia Impactor will sign the IPA and install the Chimera jailbreak app on your device.
Step 7: Launch the Settings app from your device, then choose General from the Settings menu, then tap Free iPhone space.
Step 8: Tap to select the iOS software update from the list and then click Remove Update. When asked to confirm, press the Delete button.
Step 9: Return to the General menu by tapping at the top left of the screen, then go to Settings > General > Profiles & Device Management,select your Apple ID.
Step 10: Press the Allow button “email@example.com” and choose Allow from the notice that appears, scroll up from the bottom of the screen or down from the top right to view the Control Center and activate airplane mode.
Step 11: Press and hold the power button (power on and volume up for iPhone X) and slide the slider to turn off the iPhone. After a few seconds, press the power button to restart the device.
Step 12: Enable airplane mode and launch the Chimera app, then click on the Jailbreak button to start the process.
You will be notified of the reboot request. Tap the OK button to restart the device.
After that, reopen the Chimera app and press on the Jailbreak button again. Chimera will restart and Sileo will appear on your smartphone.