How to film in the 1:1 ratio with a DSLR camera ?

I don't know of any DSLRs that shoot video at a 1:1 aspect ratio, although they may be out there.

A suggestion would be to shoot in 16:9 1920 x 1080, making sure you keep the subject in the center of the frame, then drop that into a 1:1 sequence set to 1080 x 1080 - that will center cut the middle portion of the 16:9 frame to fill the square aspect.

Make sure in Premiere you go to Preferences > Media  and set Default Media Scaling to none.

You'll just have to keep reminding yourself to keep everything important in the center of the viewfinder. Test and see if it works for you.

MtD