Jayne Dowle: Keep out of the crinoline, Kate, and focus on the reality of your romance

I hope you have had a good Christmas. This will be the last one you have spent as a single girl, at home with mum and dad. So I assume you've been slobbing about in your pyjamas, eating chocolates for breakfast and popping out to spend your gift vouchers in the sales with your sister. Next Christmas, expect thousands to turn up at Sandringham just to see what hat you are wearing to church.

Before we go any further, a word about hats. You might be relieved to hear that Joan Rivers, the American comedienne, generally approves of you, but is rather scathing about your somewhat individual choice of headgear.

Take no notice. If you like it, wear it. No-one had a more distinctive taste in hats than the late Queen Mother, and it never did her any harm, did it?

Sorry if this sounds shallow, but the clothes are very important. That navy blue Issa dress you wore for the engagement photos? Perfect. Modern, well-cut, showed off your lovely figure and copies sold out online within hours.

It strikes me that with the wedding fast-approaching in April, you haven't got much time to get your frock sorted. My mother had me touring the bridal outfitters of Sheffield a year in advance. But please don't panic and let anyone pressurise you into anything with frills, furbelows or a crinoline so wide that you can only get down the aisle in Westminster Abbey sideways.

Leave the fairy princess look to Katie Price. You're never going to be as skinny again, so choose something that makes the most of those pre-wedding nerves. And please choose a British designer. In your own understated way, you are already a style icon, and you have the potential to become the best international ambassador for British fashion since, well, Princess Diana.

Now, Princess Diana. It can't have been easy for you, taking on a man who lost his mother so young, so tragically, and so much in the public eye. Your late mother-in-law won't be at your wedding, but she will never be out of the picture.

Clearly, William trusts you implicitly to respect his feelings and to honour the memory of his mother. If he didn't, then you wouldn't be wearing her engagement ring. Not every young woman would have readily accepted such an obvious token of devotion. Not every young woman would have taken on the responsibility of a man with so much emotional baggage.

This suggests to me that you have a special understanding of how much the memory of his mother means to your husband-to-be, and this has played no small part in him choosing you to be his bride. There will be times when the pressure of living up to her and the inevitable comparisons will make you want to scream, but keep this understanding close to your heart. William obviously loves you for it.

And, as you will know already, families can be complicated. Few are as complicated as the one you are marrying into. My advice is to develop an enigmatic smile that covers all eventualities. That's what I did 20 years ago when I met my own dear husband, and I've not got into trouble yet. I'm sure that there is an army of courtiers already advising you of the protocol of what to do, curtsey-wise, if you bump into the Queen on the way to the loo in Windsor Castle in the middle of the night. But they won't tell you whether it's okay to offer to help with the washing up after lunch.

So take it from one who also married "above herself". Although infiltrating solid Home Counties middle-class stock isn't quite joining the British Royal Family, some of the rules I've picked up along the way might be useful for you. Don't show off about your holidays/high-achieving friends/new bathroom, don't buy ostentatious presents, keep your comments about the eccentricities of your in-laws for the car journey home, and should you be blessed with children, remember to call them after their grandparents/great-grandparents.

You're lucky. "Charles" and "Elizabeth" compare quite favourably with "Marjorie" and "Ernest".

Everything you do will be scrutinised, so I am very touched that you and William appear to have managed to create some kind of semblance of a normal life together in your cottage in North Wales. Where-ever you end up living, I hope you will always be able to have those evenings in together with a pizza, a DVD, and the Royalty Protection Squad boys tucked away safely in the kitchen.

We married couples all need to escape from the world, to remember what brought us together in the first place. This will be especially important to you when you find yourself stuck to your seat with perspiration while two dozen locals perform yet another traditional welcome dance on the airport tarmac in some far-away Commonwealth country.

As your lipstick melts down your chin and you get the urge to giggle, look sanguine and think of the rain lashing down in North Wales, an extra-large pepperoni on the coffee table and Top Gun on the telly.

Good luck.