It is actually quite easy to take the Apple "Basic" template - or any other, and customise it to what you want.
If you select your templete, open it - name and save it.
Slide the "Book" bar on the left down to reveal the "Layouts".
I now assume you will have assembled 1 background image plus any "placeholder" images you want to use.
Basically from now all you need do is - for example - select the background, if you do not see the grab handles, its locked... select ARRANGE top menu, if it say LOCK - you can delete or edit as you will. If it says UNLOCK - click to do it!. The inactive option is greyed out.
Change everything - change text by selecting it and changing font,size,colour etc,. and with styes drawer, you can ( using Body as example) click the down tab and click on the - "REDEFINE STYLE FROM SELECTION".
The body text throught the templete will now change to the parameteres you set.
The images you see in free templates are all in the Public Domain - but in truth they are there simply to look good in the colour scheme.
I use images from the set I will actaully be using as placeholders and set them on a blank page.
Tip - in Templates - simply copy the blank page and paste - it will drop a page "Blank 1" do again and its Blank 2 - which you can rename-select and type over.
Any thing not "working" select and open ARRANGE check to see if its locked or not.
Remember anything you need to change as you work.. like adding images, select the image not the frame and in the Inspector >Layout/Numbering. tick the layout Object box to make editable ( in the working page)
I have about 100 template now - work for idle hands! Its not really difficult - IF you take time to check everything and dont overlay things like background images.. delete teh origianl if there is one and drop or insert your own..send it to the back and start to think in layers which you stack up on the page.
Try doing some basic changes to learn slowly.