One way to do it is the way you mentioned, where you draw a smaller-than-screen-size version of your game either onto the backbuffer or onto a texture, and then either you draw that texture full-screen size or you grab that portion of the backbuffer into a texture and draw it full-screen.
However, that really is involving steps that you don't need to take. What you need to do is correctly define your viewing Frustum ie your projection matrix, with which in a single call it's possible to set up a scaled display. If you define the viewing area for a 800x600 screen as 0,0 through 400,300, for example, anything you draw in the top left 400x300 area of your screen will fill the whole screen with 2x2-sized `pixels`. Similarly if you said your projection covers coordinates 0,0 through 200,150 then you'll get 4x4-sized pixels. see the glFrustum, glPerspective or glOrtho2D commands. With this method you don't need to play with textures at all and you don't have the extra overhead of redrawing them.
However, that really is involving steps that you don't need to take. What you need to do is correctly define your viewing Frustum ie your projection matrix, with which in a single call it's possible to set up a scaled display. If you define the viewing area for a 800x600 screen as 0,0 through 400,300, for example, anything you draw in the top left 400x300 area of your screen will fill the whole screen with 2x2-sized `pixels`. Similarly if you said your projection covers coordinates 0,0 through 200,150 then you'll get 4x4-sized pixels. see the glFrustum, glPerspective or glOrtho2D commands. With this method you don't need to play with textures at all and you don't have the extra overhead of redrawing them.