How do I set up a blurred background when filming with a Canon EOS 70D + EF-S 18 55 IS STM?

The answer from Bill is correct, but does not really tell you enough. Yes, you need a large aperture (low f-stop number), but your problem is your lens. The "kit" 18=55 STM lens is not what is known as a constant aperture lens. It "stops down" as you zoom to the 55mm focal length. It closes down to f 5.6, which is not actually a large enough aperture for a blurred background, especially at only 55mm and with a wall as close as 2 - 3 feet behind you. You really need two things.

1. A lens that provides a large aperture at all times. The 40mm 2.8 STM would be far better than what you have now

and

2. You need to be much farther away from the wall.

With a 2-3ft gap to the wall you are going to struggle!

Here is how to maximise the effect

Set the camera mode dial to manual - on any other setting movie exposures are totally automatic.

Set your lens to 55mm

Switch on movie mode and set the aperture to f/5.6

Get the camera as close to you as you can and get as far from that wall as you can.

Then set shutter speed and ISO to get the correct exposure.

That is as good as it will get with that lens.

If you have £50-60 to spare, buy yourself an EF 50mm f/1.8 MkII and be amazed at the difference. Image quality is fantastic but the AF in that lens is a little "agricultural" so if you can run to another £50 or so, get the STM version. The STM will be silent during AF should you ever use it for other than fixed distance shooting.

To get a shallow depth of field you need;

- wide aperture and longer focal length (so your kit lens set at f5.6 - the widest it will go at 55mm)

- close camera to subject distance vs the subject to background distance (ie the wall needs to be MUCH further away).

Or buy a faster lens (eg 50mm f1.8 - these are not expensive).

If the wall is close to you it will not appear greatly blurred.

For most blur with you equipment, set the focal length to 55mm and the aperture to max open (probably f5.6, certainly not f11), stand as far as possible from the background, and stand as close to the camera as possible.

If you have the use of a lens of longer focal length (tele zoom lens), e.g. 55mm to 200mm or similar, then use that lens.

As with most photo effects, there is a formula for best realisation of any named effect, and your conditions are not ideal.

Position the subject as far away from the background as possible, position the camera as near to the subject as possible, zoom in, then use lowest f/number available.

Come farther away from the wall and try again.

You need a wide aperture (low f-number).