My question is how to install plugins in Minecraft single player?
Not at all. Plugins are for servers, and they fall under "multiplayer".
Just make a server on your PC.
That will not do. Plugins are meant for servers, and servers always run in multiplayer.
You can set up a local server and plugins there. In principle also have single player only that you always have to start 2 things.
Give more detail
What minecraft, what version, on which game platform and which plugin?