Probably I am late, but the best (and the most obvious) way to learn Linear Programming is exercises, with time, you will be able to distinguish different types of problems such as manufacturing, investment, job allocation, assembly, etc,,,
When you read the question, you have to imagine it, see yourself in the condition of the manager, what are the things that you have control on? (decision variables) what are the things that are preventing you from taking full control on these things? (constraints) why are these constraints being a problem? (the less or equal / more or equal part of the problem)
The more vividly you imagine it the easier it gets to formulate.