You can build a page from scratch by adding Div Tags. These hold elements on the page in a hierarchy called Document Flow
Quick reminder
in Body Insert → Div tag, name it and format it. This will be the container or wrapper
Add other Div Tags inside the container, above or below each other as required.
Float them if needed.
See Basic CSS page for notes about using CSS to format
Details
see example page that also shows links and example page How to Make a Side Bar
Start with a blank page and first make a container (or wrapper):
from Common section of Inset menu bar → 
OR Insert (top menu) → Layout Objects → Div Tag
Complete insert box: where you want the Div Tag to be
Give it an ID - use container or wrapper if it is first one on the page - this will contain all the other elements including other Div Tags. You can give it properties now, or later.
Continue to add other Div Tags by the same method - giving each an ID such as header, main content, etc. - do not have spaces in the name, it is customary to add a capital to the second word if there is one: likeThis.
Be careful about the order you put them in, eg before or after other tags. For example do not put the footer inside the main content by mistake, but put all inside the wrapper.
To add properties:
To start you may want to strip out the DW formatting that gives padding, margins, paragraph spaces, etc. To do this go to New CSS Rule and put * in the selector box. Click OK. In menu box choose Box and make padding and margins 0.
Then go on to format the container with your own requirements: font, colours, box, background, padding, etc. Everything in the container will have the same properties until you override them with separate properties of other elements inside the container.
To centre the container: In formatting menu choose box. In margin section untick Same for All, make right and left auto, top and bottom 0.
Then if you want a different colour for the header, select that and choose New CSS Rule again and give the header the properties you want.
Note that as you give properties to elements the Document Flow controls which properties apply to which element. The list along the bottom of the page shows the order - the right-most one holds its own formatting and is inside the one next to the left, etc. (The order on the CSS rule panel will be the order the rules are made in - but can be changed by dragging and should be put in the Document Flow order.)
See also Basic CSS Instructions page.
To add and float an image within a Div Tag:
Insert the image in the vertical position you want it. Select it and choose New CSS Rule. In menus window → box → float, then choose left or right. Add any other formatting such as padding or margin, etc.
To make a Div Tag into a Side Bar
Have enough tags to make what you need, called, eg container, header, sidebar, main content, footer. To start they will be inserted as above.
Select the sidebar and New CSS Rule, in formatting menu box choose Box and give it a width then → float → left
Then select main content, New CSS Rule, give it a width and float it right
Then select footer, New CSS Rule, and in Box → clear → both
This should put the sidebar and and main content side by side and the footer below.
You can decide whether to adjust the widths of the sidebar and main content so they meet, the two widths should not add up to more than the wrapper width or else they will not go side by side.
If this does not come out how you expect it may be because you have not put the Div Tags in the right order to start with.