Adjectives come after the noun in Welsh.

Good is da
Morning is bore
Good morning is bore da

Afternoon is pnawn
So Good afternoon is pnawn da

Good coffee is coffi da
A good car is car da

One small complication is that Welsh nouns all have a gender, they're either masculine or feminine.

Bore, prynhawn, coffi, car and gwin are all masculine, but the word noswaith (evening) is feminine, and this will cause a letter change - a soft mutation - in the describing word, so good evening is noswaith dda.

Ffilm is also feminine, so a good film will be ffilm dda.

Gêm is also feminine, so a good game will be gêm dda.

Unfortunately there are few clues to help you know what's masculine or feminine, but you will still be understood if you forget to mutate after feminine words.