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I started with a pair of plain black off-brand Chuck Taylor knockoffs. (Boo Nike!) These lack the chucks' distinctive white disc logo at the inner ankles. They were just unadorned black canvas.
I traced around a 2" jar top with a silver Sharpie, lightly sketched the seal freehand in black pen using the Seal of Rassilon on my screen as reference, and filled it in with white fabric paint using a very fine-tipped paintbrush. If you'd like to do this, you could probably make your life easier and your result cleaner by using a stencil of some sort.
While the genuine articles have their logo only on the insteps, I chose to do the outsides as well.
If your choice of footwear already has logos on, you could simply paint over the logos completely with the white fabric paint (which incidentally is what is done with David Tennant's costume shoes, as they aren't allowed to show the product logo on-screen) and draw your seal on top of that with a dark marker or dark paint.
If dear old Rassilon is too old-school for you, you could use the new Time Lord seal instead.
Thanks for reading!