"Quicktalk" is a plugin for Blender which tries to do most of the dull boring work for lip-synch for a character.
There's a video tutorial on the addon's website here: http://tentacles.org.uk/quicktalk
First, make shape keys for the mouth shapes. You'll need: AI, E, O, U, FV, L, MBP, WQ, ETC and the Basis should be a rest shape.
Give it an armature if you don't already have one. It needs an armature to attach to the shape keys.
Put the 3d cursor near the object's face. It's where we'll create the panel of bones to control the shape-keys. Click the "Build Shape-Key Panel Armature" button. That'll build a panel of bones that control the shape-keys Each bone can be rotated from 0 to 90 degrees like a lever to control the expression. It'll build a lever for EVERY shape key so you can include blinks or smiles or whatever to get a lever for them too.
Select a script file. Script files contain the dialogue the charters in the scene speak. Lines with a single word followed by a : mark changes in which character is speaking. The armatures attached to the character should be named the same as the name in the script. Well, first few chars have to match anyway. Lines which aren't a single word followed by a character are spoken by that character. Example:
Monkey: Hello I'm a monkey Welcome to my cartoon
Dog: Yes, and I'm a dog.
Monkey: Welcome to my cartoon dog.
Click the "Guess Dialogue Markers" button. This will try to guess where in the timeline there's a change in who speaks and put markers there. It will be wrong, so adjust them so they are right. You can make them yourself if you need to for some reason, they are just normal markers but with names ending in "!D"
When they're right click the "Guess Line Markers" which will add markers for each line that each character speaks. Again they'll be wrong and you'll have to adjust them. They end in "!L"
When they're right click the "Guess Word Markers" button. It'll put markers for every spoken word. There's more chance this will be more or less right but still not much chance so fix 'em. Word markers end in "!W"
Next you need to pick a dictionary that describes which phonemes correspond to which words. You can find a dictionary at the The CMU Pronouncing project here:
http://www.speech.cs.cmu.edu/cgi-bin/cmudict
Once you've downloaded a dictionary, click the browse button to tell the plugin where that is
Select an armature and click "QuickTalk Plot" This will create f-curves for the scene which change the x-rotation of the bones created in the panel. There's a pretty good chance this will look all right, but especially long words or where there's pauses in speech it'll still be wrong and you'll need to squash and scale the f-curve to match.
If all that sounds long-winded, you do not understand how damn tedious lip-synching is when you have to do it literally frame by frame.
There's the original Quicktalk.py that you can install on blender before 2.8.
There's a Quicktalk28.py for version 2.8 up to version 4.
Quicktalk4.py is all you need for Blender 4 onwards.
Just click "edit/preferences/Add-ons/Install and then pick the appropriate Quicktalk python file for your version of Blender. Remember the check the box to actually activate the installed plugin.