Partial Class Winform Form

It was taught to me that any alterations to an object on a form (change the text, make visible/invisible or change the color and so on) should be done in the corresponding form class.

But due to the large amount of such alterations done inside the project, the file has became large and hard to search through. I've read online that a Partial Class could help, but there was no explanation on how to implement this. As an easy example, I have the following 2 files: Form_Main.cs

namespace Test
{
    partial class Form_Main : Form
    {
        public Form_Main()
        {
             InitializeComponent();    
        }
    }
}

AND Form_Main.Dataloader.cs

 namespace Test
    {
        partial class Form_Main : DataLoader
        {
            public void SetText()
            {
                TextBox_StudentSurname.Text = "1";        
            }
        }
    }

How can I make this work? Because if I do this I get several errors in the designer.