Tiling windowsill

[Grendel]

I'd probably trim it flush to the wall and then tile over it. Looks like you have plenty of thickness on the window frame for it not to look weird.

How would I get the tiles to stick to glossed wood?

Rough up the surface with coarse sandpaper or even put down a layer of thin ply on top then tile as normal.

[Rorschach]

I'd probably trim it flush to the wall and then tile over it. Looks like you have plenty of thickness on the window frame for it not to look weird.

How would I get the tiles to stick to glossed wood?

Sand it back a bit and then just glue them on with construction adhesive, they are only decorative after all.

I'm going to do a tile backsplash in my kitchen next year, I'll just be using that same method to put them on the wall, it's only a single row of tiles.