When your bouquet arrives, some of the flowers will be 'in bud'. Firstly, cutting the stems by a couple of centimetres at a 45 degree angle before putting in your vase will help them get a big drink and encourage the buds to bloom fully over the next few days.
You can very gently roll carnations between your finger on the soft green bit where the petals meet the stem, and this will encourage them to open even wider.