First question would be: Just what are we talking about? there are a *lot* of junctions under the sink. Which one of the many is leaking? If it is unclear which one, first step would be to identify the source. Fixing one when the source is someplace else is time and effort wasted.
The most likely source is the sink to drain. That is in no way to suggest this *is* the source in this case. If it is the source, then tear down / clean / new putty is the first step. Second step it to tighten things. It's an "enough but not to much" sort of process. If you redo it and get a leak either it's to tight or not tight enough. Try tighter and see if that fixes it. If it doesn't then it was to tight at the first attempt ( welcome to your training as an apprentice plumber ....).
There are a number of possibilities. Each has it's tricks and issues. Going into them all would take a bit ( even for long winded Bob