If it is a twin bed length ways against the wall like a sofa you can by a day bed set that is a standard twin and have a attached skirt and have that tucked look. I thinking tucking is a hassle day to day. lol

I bought a set of linen sheets which have now had two trips through the washing machine, a cool wash 30C and no fabric softener and they are lovely, soft and comfortable to sleep in. Never wash your linen in hot water.

It depends on the thickness of your mattress. I have a fairly standard height king mattress, on a ensemble base. I bought a standard AU king duvet and it shows a lot of mattress. Then bought a West Elm US king duvet and it's much larger. It covers the mattress, tucking in would be possible but your not able to tuck in a lot of fabric. I love the larger West Elm duvet, but they weren't selling well as the sizes were different from our standard, so they now only do smaller AU sizes. And the US West Elm site won't sell to AU... So I think if you want a generous duvet or quilt to tuck in. You may need to go custom made.

I love shades of blue through to green, so I'd take my cue from the deep green-blue throw rug in the Adairs photo. Preferably in a different fabric to velvet to mix up textures a bit. I do like the Morgan and finch diamond cushion too, if you prefer a more colourful option.