Very accurately! The eye of the viewer can detect any irregularity in a shape that it fully understands. A "square" that does not have sides of equal length, a "circle" that is not circular and an "ellipse" that deviates from perfection is immediately noted as being not what it claims to be - or, more precisely, it is not what the eye expects to see in that situation and is detected as being "foreign". In short, a "circular" circle is accepted in passing and the overall reality of the scene is maintained. The same is true of the ellipse.

So far we've just drawn our ellipse as a circle distorted within a square conforming to vertical perspective. But for horizontal ellipses to work correctly you must also take their distance above or below the horizon into account. If you hold up a can of beans in front of you and lift it above your eye-level (the horizon) you will be looking under the can; below your eye-level and you will see it from above. Hold it straight in front of you and you will see the top half of the can from below and the bottom half from above. OK?

Once you understand the basic geometry of the curves involved you will probably find it sufficient to only lightly draw length and width dividers and add points marking the required dimensions.

If you need to draw really large ellipses it will often assist you to draw a smaller version first - small enough so you can see the whole ellipse without having to move your eyes. You can then enlarge this onto your final drawing surface (using any means you prefer from projection to mechanical resizing) or use it as a visual template to guide you when you draw it full size.

Finally there will be times when you need to draw one ellipse (or more) inside another. For example, when drawing a round, hollow object with walls having a substantial thickness, such as a terracotta flowerpot.  In this particular case you need to draw just the top half of the outer ellipse and to shift the inner ellipse slightly higher to account for perspective (the rear wall is further away than the front so a is narrower than b). The lower half of the outer ellipse needs to be "seen" only in your mind as it is solely defined by the lighting and shadow of the front face. If you attempt to draw this line first, however lightly, you stand a good chance of it remaining to haunt you - even after you've erased it, its slight indentation will probably attract extra graphite from your shading and be forever present!