SQL Server Baseline and Performance Optimization
Here are some good articles with some practical examples that you can find here: 

How to detect SQL Server performance issues using baselines – Part 1 – Introduction

How to detect SQL Server performance issues using baselines – Part 2 – Collecting metrics and reporting

How to detect SQL Server performance issues using baselines – Part 3

While Part 1 will provide you some basic knowledge about what the baseline is, in Part 2 you can find info how to do that on your own using "poor man" method

Part 3 provides some examples on how you can establish baselines and how to use baselines in troubleshooting some issues via ApexSQL Monitor