An example plugin class that uses ProtocolLib to show a sign with predefined text

SignTest.java

public final class SignTest extends JavaPlugin {
    private ProtocolManager manager;

    @Override
    public void onEnable() {
        manager = ProtocolLibrary.getProtocolManager();
    }
    
    @Override
    public boolean onCommand(CommandSender sender, Command cmd, String label, String[] args) {
        if (sender instanceof Player) {
            Player player = (Player) sender;
            sendSignEdit(player, "Hello", "there", "my", "friend");
        } else {
            sender.sendMessage("This command can only be executed by a player");
        }
        return true;
    }

    public void sendSignEdit(Player player, String... texts) {
        BlockPosition position = new BlockPosition(player.getLocation().getBlockX(), 255, player.getLocation().getBlockZ());

        PacketContainer blockPacket = manager.createPacket(PacketType.Play.Server.BLOCK_CHANGE);
        blockPacket.getBlockPositionModifier().write(0, position);
        blockPacket.getBlockData().write(0, WrappedBlockData.createData(Material.SIGN_POST));

        PacketContainer dataPacket = manager.createPacket(PacketType.Play.Server.TILE_ENTITY_DATA);
        dataPacket.getBlockPositionModifier().write(0, position);
        dataPacket.getIntegers().write(0, 9);

        NbtCompound compound = (NbtCompound) dataPacket.getNbtModifier().read(0);
        for (int index = 0; index < 4; index++) {
            compound.put("Text" + (index + 1), "{\"extra\":[{\"text\":\"" + texts[index] + "\"}],\"text\":\"\"}");
        }
        dataPacket.getNbtModifier().write(0, compound);

        PacketContainer editorPacket = manager.createPacket(PacketType.Play.Server.OPEN_SIGN_EDITOR);
        editorPacket.getBlockPositionModifier().write(0, position);

        try {
            manager.sendServerPacket(player, blockPacket);
            manager.sendServerPacket(player, dataPacket);
            manager.sendServerPacket(player, editorPacket);
        } catch (InvocationTargetException e) {
            e.printStackTrace();
        }
    }
}