package aeronicamc.mods.mxtune.events;

import aeronicamc.mods.mxtune.Reference;
import aeronicamc.mods.mxtune.init.ModBlocks;
import aeronicamc.mods.mxtune.init.ModItems;
import net.minecraft.util.text.TextFormatting;
import net.minecraft.util.text.TranslationTextComponent;
import net.minecraftforge.api.distmarker.Dist;
import net.minecraftforge.event.entity.player.ItemTooltipEvent;
import net.minecraftforge.eventbus.api.SubscribeEvent;
import net.minecraftforge.fml.common.Mod;

@Mod.EventBusSubscriber(modid = Reference.MOD_ID, value = Dist.CLIENT)
public class RenderEvents
{
    @SubscribeEvent
    public static void event(ItemTooltipEvent event)
    {
        if (event.getItemStack().getItem().equals(ModBlocks.MUSIC_BLOCK.get().asItem()))
            event.getToolTip().add(new TranslationTextComponent("tooltip.mxtune.block_music.help").withStyle(TextFormatting.YELLOW));
        else if (event.getItemStack().getItem().equals(ModItems.MUSIC_PAPER.get()))
            event.getToolTip().add(new TranslationTextComponent("tooltip.mxtune.music_paper.help").withStyle(TextFormatting.YELLOW));
    }
}

package aeronicamc.mods.mxtune.events;

import aeronicamc.mods.mxtune.Reference;
import aeronicamc.mods.mxtune.init.ModBlocks;
import aeronicamc.mods.mxtune.init.ModItems;
import aeronicamc.mods.mxtune.util.IInstrument;
import aeronicamc.mods.mxtune.util.SheetMusicHelper;
import net.minecraft.client.Minecraft;
import net.minecraft.client.gui.FontRenderer;
import net.minecraft.entity.player.PlayerEntity;
import net.minecraft.item.ItemStack;
import net.minecraft.util.text.TextFormatting;
import net.minecraft.util.text.TranslationTextComponent;
import net.minecraftforge.api.distmarker.Dist;
import net.minecraftforge.client.event.RenderGameOverlayEvent;
import net.minecraftforge.event.entity.player.ItemTooltipEvent;
import net.minecraftforge.eventbus.api.SubscribeEvent;
import net.minecraftforge.fml.common.Mod;

@Mod.EventBusSubscriber(modid = Reference.MOD_ID, value = Dist.CLIENT)
public class RenderEvents
{
    private static final Minecraft mc = Minecraft.getInstance();
    @SubscribeEvent
    public static void event(ItemTooltipEvent event)
    {
        if (event.getItemStack().getItem().equals(ModBlocks.MUSIC_BLOCK.get().asItem()))
            event.getToolTip().add(new TranslationTextComponent("tooltip.mxtune.block_music.help").withStyle(TextFormatting.YELLOW));
        else if (event.getItemStack().getItem().equals(ModItems.MUSIC_PAPER.get()))
            event.getToolTip().add(new TranslationTextComponent("tooltip.mxtune.music_paper.help").withStyle(TextFormatting.YELLOW));
    }

    @SubscribeEvent
    public static void event(RenderGameOverlayEvent.Post event)
    {
        PlayerEntity player = mc.player;
        ItemStack itemStack = player.inventory.getSelected();

        if (event.getType() == RenderGameOverlayEvent.ElementType.ALL && (mc.screen == null) && itemStack.getItem() instanceof IInstrument)
        {
            FontRenderer fontRenderer = Minecraft.getInstance().font;
            ItemStack sheetMusic = SheetMusicHelper.getIMusicFromIInstrument(itemStack);
            fontRenderer.drawShadow(event.getMatrixStack(), SheetMusicHelper.getFormattedMusicTitle(sheetMusic), 10, 10, -1);
        }
    }
}

Test in-game gui overlay rendering.