How to make custom server control in asp.net
First draw picture of the custom server control in mind, after make custom control. Suppose we have a picture and we want to make server control of that picture.
Follow some steps for creating custom server control in asp.net

Step-1 : Open visual studio
Step-2:  Goto file-->new-->project-->select web in left pane-->select asp.net server control in middle pane
Step-3:  Write name of your project as 'customserver_control' and click on 'ok' button
Step-4:  After click on ok button you can see 'ServerControl1.cs' file is appear
Step-5:  Change  c# file name as 'customtextbutton' control
Step-6:  Remove this code, which is mentioned below from your c# file

[Bindable(true)]
        [Category("Appearance")]
        [DefaultValue("")]
        [Localizable(true)]
        public string Text
        {
            get
            {
                String s = (String)ViewState["Text"];
                return ((s == null)? "[" + this.ID + "]" : s);
            }

            set
            {
                ViewState["Text"] = value;
            }
        }

        protected override void RenderContents(HtmlTextWriter output)
        {
            output.Write(Text);
        }

Step-7: Replace your code with mentioned below code.
[ToolboxData("<{0}:customtextbutton runat=server></{0}:customtextbutton>")]
Note: class name or tag name are same

Step-8: Design CreateChildControls() override method in the class with some statements, which is mentioned below.
Step-9: Now,Your code look like

Label l1:
        TextBox User_txt;
Label l1;

protected override void CreateChildControls()
        {
                l1=new Label();
                l1.ID = "label1";
                l1.Text = "User_Name";
                User_txt = new TextBox();
                User_txt.ID = "usrtxt";
                User_txt.Text = string.Empty;
this.controls.add(l1);
this.controls.add(User_txt);


        }

Step-11: Take override  Render(HtmlTextWriter writer) method  for rendering the object onto the browser
Step-12: write this code inside this method
protected override void Render(HtmlTextWriter writer)
        {
            l1.RenderControl(writer);
            User_txt.RenderControl(writer);


        }

Step-13: Build this project

Now how to use .dll or .exe file in your web application

Step-1: click file-->new-->project-->selct web in left pane-->asp.net web form
Step-2: Right click on tool box and choose new item
Step-3: Browser your project .dll file and add your control onto your toolbox.