tag How do you branch and merge with TortoiseSVN?
trunk branch tag svn best practices (3)
How do you branch and merge with Apache Subversion using the TortoiseSVN client?
My easy click-by-click instructions (specific to TortoiseSVN) are in Stack Overflow question What is the simplest way to do branching and merging using TortoiseSVN?.
You can also try Version Control for the Standalone Programmer - Part 1 or perhaps Merging with TortoiseSVN.
A very good resource for source control in general. Not really TortoiseSVN specific, though.