By taking the time to sketch, or plan, your layout first you can change your mind as many times as you like before permanently fixing something in place. Click on the link to see how I utilized scrapbooking sketches for one special photo.
Don't worry, you don't need to be able to draw neatly to create a sketch. Arm yourself with a pencil, ruler and eraser and you are all set, although you might like to consider adding a photo stencil to this toolkit, but I will talk more about below.

Featured here is a sketch and a layout created from it
Start with a piece of scrap paper, either the actual size of your layout, or half of those dimensions - if you start with the smaller piece, remember to reduce the size that you draw the elements.
Decide on which photo(s) you want to use, taking into account both the content of the pictures and their shape. You may wish to use all landscape photos, or all portrait, for example. I would suggest starting with just one or two photos per page for your early scrapbook sketches.
What I like to do at this stage is to take a piece of clear acetate and cut a photo-sized hole in it. This then acts as a stencil which I draw around, to mark the position of my photos on the page. Because it is clear, I can see where the existing elements of the page have been placed in my plan, before adding the next. I can turn it so that I can add landscape or portrait photos, making it very versatile.
Pick a pretty thick plastic sheet and your template will last you for ages - I like to use the sheets sold as report covers.
You can also create a second stencil which has holes that are slightly larger, to allow for the mat around the photo. It is useful to mark this template with a permanent marker, so it is easy to tell at a glance which is which.
Using a soft pencil (an HB or B would be ideal) lightly draw around the inside of your smaller template to position your first photo on your scrap paper. Then lay the larger template on top and mark where the mat will be positioned. You might like this to be the same distance from the photo all round, or you may wish to leave a slightly larger gap top or bottom. You are not limited in any way here, of course. If you want your mat to be "skew whiff" then go ahead! It's your layout, after all.

To help make it easy to see which shape on your plan represents your title, it is common to write TITLE in the space you have allocated for it.
Likewise your journaling shape would normally have a series of lines drawn within it to suggest rows of text. There is no need to be accurate here, and draw exactly the right number of lines in your sketch, they just act like a label to indicate that the journaling will be placed inside that shape.
Again, there is no need to draw your embellishments accurately on scrapbook sketches. Simple circles, rectangles, triangles or squares will suffice.
Position them on your layout, grouping if required, trying to keep the whole thing balanced. Odd numbers often work better than even, if you have repeating elements in your design. So for example add three roses, with perhaps two in a group and a third on the other side of the page.
What you add, of course, will depend on what embellishments you have to hand or can make.
This is where scrapbooking sketches really prove their worth. If you layout all your elements on the page and then realize that a border is necessary, it is easy to add it on your sketch, by moving other pieces. If, however, the existing elements were already stuck down, it might not be possible without messing up what you have done so far.
You can also buy books and ebooks full of sketches that you can use, which is quicker than creating your own. My favourite is 500 Scrapbooking Sketches by Jennifer Gormly.