These tips are for blogger/blogspot users. I did the big template change today...and thought others might be curious on the how-to. And I wanted to share before I forgot how to do it. You never know, I might need to refresh my memory at some point :)
First step is to download the new template--whichever one you choose--granting that there are no structural problems within it--this should be relatively easy. You can try clicking on it or doing a "save link" or "save link as" option. The file should end in .xml I think. At least that is what the two I've downloaded ended with.
2) Once it's on your computer, you'll need to go to your blogger dashboard and click on "layout." Then you'll need to click on "edit html."
3) You'll have the option to upload a template from your hard drive. You have "browse/upload" buttons. Click on browse. Choose the template you've downloaded. Choose "open" to select the template. Then click on upload.
4) You'll probably receive a message that you're losing widgets. Essentially ignore this. BUT I should have probably mentioned this before. You will want to save any of your sidebar content. What I would do, if I were you, would be to simply copy that content and paste it into a blog post. ("Create new post, paste the information, click on save draft.)
5) Save the changes you made to the template. Your page should say something to the effect that your changes have been saved and giving you the option to view your blog.
6) In the rare case (or maybe not so rare) there may have been a problem with the html of the template. I tried four different templates. Two worked just fine. Two had problems with the coding. I went with the ones that worked. I wasn't about to try to figure out what was wrong with the others. So you might download two or three of your favorites so you have options in case your first choice doesn't work.You will know you have a problem if blogger tells you so. I got an error message. If you don't get an error message then everything is fine and you've got nothing to worry about.
7) You can customize your template by going to your blogger dashboard, clicking layout, clicking "page elements." I think it is automatically on "page elements" unless you have it open to one of the other options. Here you see "add a page elements" sprinkled all over the place. On the top, on the bottom, on the side columns. You can click on "add a page element" to get a pop-up window. You'll be given options for adding text boxes, html boxes, list boxes, link boxes, pictures, slideshows, etc. The most common add-ons you'll probably want to use are "pictures" and "text boxes" and "link boxes" These are fairly self-explanatory. The pictures will ask you to upload a picture file from your computer or provide the url to a photo/picture on the web. Always choose the "shrink to fit" option. Link boxes will have a place for you to enter titles and urls. And text boxes are just like blog posts. Same design.
You can drag these page elements around. Up and down, side to side. Experiment at this stage until you've got it just the way you want it. You can preview your changes by clicking on "preview." When you've got everything just the way you want it...click on save. It will confirm that you've saved and tell you to view your blog.