Tell the story of what times were like when this little person came in to the world and what their family and friends were doing at the time.
Let your family and friends tell this young person in their own words and pictures about their special day, so that in years to come they may look back and know what was going on when they were Christened.
As with any story it has a start, middle and end. Bear in mind when creating this, that the subject person may not have any recollection of this day due to their age and so may only relive it thanks to your story and it may be some 10 years or so before they read it and have an understanding of the story and what it meant at the time. So you may want to start your story with telling and showing them what was happening in the world on their special day in a page or two.
The man part is obviously about the Christening, but maybe try to include words and pictures of who was there and just as importantly why they were there and what relationship they had to the person being Christened. Who the vicar/priest was, how many people were there etc.
The end of the story for this event may be about what you hope for them for the future, what you think they may grow up to be/do etc and/or just a closing note from you on the last page.