Your brain has a fantastic ability to mentally record down many stuff you've come across. It's a matter of accessing those mental files...
Simply put down what you know first. Waiting for everything may keep you waiting for a long, long time and you may never even get started.
Your brain will start digging into those files of memory and out of the blue, it may just locate the perfect piece of information that you've totally forgotten about.
One consideration though. You might want to think about people who will read your scrapbooking journaling in future. They weren't there with you, so they might be oblivious to the facts.
Making a habit of noting down as much details as you possibly can, will help paint a vivid story and picture for them. Bring them along with you in reliving those memories of yours.
Basically, write straight from your heart.You don't have to restrict yourself to complete your layout and scrapbooking journaling all at once.
In fact, many a times, I find that I'll be doing a layout for my page first, then allow space for my scrapbooking journaling and move on to the next page and then next. After I'm pretty done up with my layouts or I've gathered a neat pile of layouts, I'll go back and fill in my scrapbooking journaling for all the pages at one sitting.
A good guideline is to leave about half a photo space per page for your scrapbooking journaling. Unless you're intending to do a full-blown story, that should be sufficient space.