"These men are grumblers and faultfinders; they follow their own evil desires; they boast about themselves and flatter others for their own advantage." NIV translation
In this verse, Jude gives us his final three signs of these men and their false teachings. First is that they don't have the joy of the Lord in them. As Jude describes, they find fault with everything and are not content with anything. Second, they do not seek God's will in anything as they simply walk in the flesh instead of being led by the Spirit. The final sign is that they smooth talk others and tell people what they want to hear so that they can profit from it.
"But, dear friends, remember what the apostles of our Lord Jesus Christ foretold. They said to you, 'In the last times there will be scoffers who will follow their own ungodly desires.' These are the men who divide you, who follow mere natural instincts and do not have the Spirit." NIV translation
Jude reminds the readers here that these false teachers are not a big surprise as the apostles have previously warned us of their coming.
They divide people because they do not have/follow the leading of the Holy Spirit and therefore are blown along by their own desires.
"But you, dear friends, build yourselves up in your most holy faith and pray in the Holy Spirit." NIV translation
Now that Jude has made us aware of the false teachers and told us how to spot them, he tells us what to do to stand in the faith. He tells us to "build yourselves up" and he is talking about reading and studying the Word of God that is contained in the Bible. That also means ALL of the Bible and not just a few topics that are regularly touched on at "churches" these days. The second thing is to "pray in the Holy Spirit" and what he is talking about is not the typical prayers of going to God with a wish list of what we want from Him. Jude is talking about praying and listening to the Spirit to help us to know the will of God as well as some serious worship in prayer and thanking God for all that He has done. In the armor of God (see Ephesians 6:10-18) this prayer is one of the offensive weapons (sword of the Spirit) we are given to fight the enemy with. We should take note here that Jude mentions studying the Word first and that is significant in that bible study helps us learn how to use that sword. You can do a lot of damage to yourself and to others by swinging around a sword without the knowledge of how to do so.