How to Install A Theme
This theme tutorial is for the tumblr customize page as of January 2014. This tutorial applys to all of my themes and all Homemade Themes except those which can be installed via the themes garden. I recomend using Google Chrome or Firefox as they are the most compatible with all tumblr themes.
Note: Circled red arrows in screenshots indicate where to click.

Step 1:

Click here to go to the customize page. Alternatively you can click on the customize button on your blog.

Step 2:

Click on 'edit html' this will show your current theme code.

Step 3:

Remove all the text in the html box: You can select all text by holding CTRL+A for pc-users and for mac-users you hold CMD+A.

Step 4:

Paste the theme code into the empty edit HTML box. To find a theme code, start at the homepage and select a theme that you like. You will then be taken to the indervidual page for that theme which features a slide show, links to live previews of that theme and a short description. You will find a button 'Install Theme' click on it and you will be taken to a pastebin page.

Step 5:

On the Pastebin Page the easiest way to copy the whole code is to click on the clipboard (indicated by red arrow). All New and updated themes have a note which states "END OF THEME" to indicate the end of the code.

After you click on the clipboard a message will appear as shown in the screenshot 'TEXT BELOW IS SELECTED. PLEASE PRESS CTRL+C TO COPY TO YOUR CLIPBOARD. (⌘+C ON MAC)'

Step 6:

Paste Code Directly into the empty edit HTML box. Then click the green Update Preview Box.

Step 7:

After completing Step 5 the customize page shoudd show be displaying your new theme. Play around with custom colors, sidebar/header/cover/background images, and other options to tweak the theme to suit your liking. I recommend using a white or light colored background. Make sure your text is clearly visible and remember the KISS Principal (Keep It Simple Stupid).