IOC Container - Composition Root

Hi Guys and Gals,

I've recently been dabbling a bit with IOC Containers (LightInject in my case).

I've been reading that you should only need to use the container ONCE, on startup, and no where else. This is what I'm struggling to understand. If I can only reference the container in a bootstrap/startup method, how is it possible to resolve what I need, elswhere in the project, or at runtime if the class depends on user input.

So In my Traditional Windows Forms App, on Form Load Say, I would Bootstrap Lightinject as per the attached file. It's only an arbitrary example, it's more the premise I need to get my head around.

I might be missing something here entirely, or just not getting it. But how am i supposed to resolve dependancies, If i can't use/not supposed to reference or use Container.GetInstance/Resolve/{Choose IOC Syntax Here}, and only in the composition root.

For Instance, Say I have two buttons and a TextBox on my form. The first button gets me an ILoader (attached file), and the second button loads up a file viewer (ILoader, attached file), whose file name is what is entered into the textbox.

Without An IOC Container I would do the following (let's just assume its put in the click event)

Button 1 would call ISplitText MyStringFunc =  new WhateverImplementsIt();

Button 2 (gets the file reader based on textbox input)

ILoader MyLoader = new FileReaderImplementation(TextBox1.Text);

Using LightInject, I'm surely compelled to do the following:

Button1:
ISplitText Splitter = Container.GetInstance<ISplitText>();

Button 2
varLoaderFunc = Container.GetInstance<Func<string, ILoader>>();
ILoader l2 = LoaderFunc(TextBox1.Text);            

Am I Incorrect? In A large project I would have Container.GetInstance<IFoo>, peppered all over the place, in the main form file and elsewhere surely, so how can i only reference the container in ONLY 1 spot, in the form of  bootstrap, am i missing a magic piece of the puzzle?

In all the sample apps I have seen it's all done in one simple console app, in the Main function. All these apps follow the format of:

Container = new Container()
Container.Register<IFoo,Foo>();
Container.Register<IBar,Bar();

var Resolved = Container.GetInstance<IFoo>();

Well, I understand all that, and it's extremely simple. It's once you start adding a bit of complexity to the app itself, I'm lost as to how to get the instances without making the Container itself public, or static, or accessibly in some way,shape or form and then calling Container.GetInstance in a million places (which apparently, is a big no no). PLEASE HELP!
Cheers,

Chud

PS - I am not concerned about "abstracting the container" itself. so would prefer to only focus on my understanding of the above
BootStrapIOC.txt