Plans are established by counsel.
v.18 Use wise guidance. Establish your plans based on good counsel and wise guidance, see 15:22. Only wage war when you have wise advice on how to do so (and whether to do so), see 24:6 and Lk 14:31. There is wisdom in the consensus among leaders.
v.19 Slander. Do not associate with a babbling fool who goes about revealing secrets (gossip), see also 11:13. The less contact you have with a gossip the better off you are. If he is willing to talk to you about others, then he is willing to talk to others about you.
v.20 Father and mother. If you curse father or mother your light will be snuffed out. This echoes the commandment given in Ex 21:17. See Jesus’ teaching in Mt 15:3-7.
v.21 Inheritance. An inheritance gained early in life may not be a blessing in the end. See the example of the prodigal son in Lk 15:11-32.

Read Proverbs 20:18-21