Simple magnetic knife bread slicer guide to slice bread at 10,12,14,16 or 18mm thick.  18mm thick is shown in main photo.

Requires 4 x 8mm Diameter x 3mm Magnets that hold the guid to the knife while slicing. I put a small drop of superglue on magnets before pushing in to hole.

Bread knife needs to be at least 170mm x 25mm so the guide can stick to the knifes blade.

knife in photo: https://www.thewarehouse.co.nz/p/living-co-stainless-steel-bread-knife/R2944543.html

Put guide on knife, align plastic guide to end of bread, apply a little pressure to keep guide on the end of bread and at a right angle and slice lightly, at bottom the guide will slide up when it hits the bread board to cut slice off, repeat as necessary. A sharp bread knife like the one in picture works great.

Note: I also added some guides for different thicknesses. 10,12,14,16 and 18mm slices depending upon model.