To make small, round shapes out of cardboard use an office hole punch (available also with funny hole shapes nowadays) or better still a DIY punching tool used to make holes in leather belts and the like
Usually after assembling models, particularly plastic ones, we are left with a handful of spare parts. Don't get rid of them as they may one day become a glorious addition to a new terrain piece or converted model
Taking down notes seems to be a must when you have hundreds of paints to choose from. But it takes time and the notes can be easily lost over time. There is another, quick and secure way however: snapshots taken with your phone that are kept in a dedicated album.