How do I add sliders and buttons?

I want to add 3 sliders to the 'graphics options' menu, but I can't find any tutorials or other documentation for this (besides net.minecraft.client.gui.GuiSlider.java)

Read some vanilla code that uses a slider.

I understand how minecraft does it, but how can I do it without modifying minecraft source code (to increase compatibility with mods like optifine and such). Is there an API somewhere?

// From iChunUtil
@SubscribeEvent
public void renderTick(TickEvent.RenderTickEvent event) {
    if(event.phase == TickEvent.Phase.END) {
        if (Minecraft.getMinecraft().currentScreen instanceof GuiOptions) { //or whatever screen you want to add the button to
            GuiOptions gui = (GuiOptions)Minecraft.getMinecraft().currentScreen;
            // Draw your buttons here using the gui
        }
    }
}

https://github.com/iChun/iChunUtil/blob/master/src/main/java/ichun/client/core/TickHandlerClient.java