"How blessed is the man who does not walk in the counsel of the wicked, nor stand in the path of sinners, nor sit in the seat of scoffers"
stand in the path of sinners
The Septuagint translation is: is not on the threshold of sinners.
The path of sinners here is the beginning of the things that sinners do. If you find yourself surrounded by those who sin, who seek pleasure over principle, who selfishly push themselves over others, who ignore or purposely violate God's boundaries, then you will be sucked into this world eventually if you stay there surrounded by it.
To be perfectly blunt, David is saying when you look at the parties and gatherings that sinners go to and you decide to be amongst them, you are unwise and will pay a price for your association. It is not as bad as doing what they are doing, but you will be learning how to want and practice what they are doing. It is better to stay out of the whole lifestyle that they live. Their lifestyle is a culture, a worldview, a 3-D model of how to live life with the values that they have. These values are not the proper ones, so you do not want to be filling your head with these images and choices and problems and paradoxes. Get clean away from the lifestyle of those who violate God's boundaries.
sit in the seat of the scoffers
The Septuagint translation is most interesting: does not sit in the plague of the sinners. The idea here is that there are people who do not do anything but they can tell you why whatever good thing you have planned will not work. They are scoffers that can pick apart and mock anything good. They are sitters who do nothing but criticize. Do not take a seat with them. Do not become one of them for your whole life will be about the negative perspective in life. If you embrace a negative tone and point of view, it will be very difficult if not impossible to turn this way of seeing the world off.
This is the third of the major life traps that seems like it is okay but twists and distorts your life as you live it. First, there is the trap of the guidance of those who are immoral with no thought of repentance or change. Second, there is the trap of those who tell themselves that they are only going to be with the sinners and will not partake themselves. Third, there is the trap of those who want to appear as experts and smarter than everyone else so they point out the flaws, the mistakes, the negative in everything. This final group adds nothing but their clever negativity, and it produces a cloud of pessimism around them that eventually becomes the air they breathe.
If you have noticed that you have started to become a negative Nelly, then stop. Don't keep doing it. State the positive about the people, institutions, and relationships in your life. If you really have a hard time with this, then you might be more of a pessimist than you think you are. Do not lie to yourself that you will applaud when they get it right. This usually means perfection and that is never reached. Dig for the reasons to cheer and only share your negative when it is asked for.
You and I live in the world that our mind tells us is the real world. Therefore it is essential that you fill your mind with the facts of the Scripture or you will begin creating a false world with false values and missing truth. Sing the songs of God and meditate on the truth of God so that your experience will match up with reality. There is a God and there will be a final state after death.