What they mean by "Do you know what you're doing?"

 Even though the question, when asked, of ,"Do you know what you're doing?" is often used in a condescending or insulting way by a lot of people, there actually is a purpose to the question when asked by people in a situation where they don't know exactly how to ask what they really want to know and aren't meaning to be insulting. 

It's actually 2 questions in most cases.  "What is the procedure to use or be used here?" and "What is the methodology being used in this project?"

Most people think that procedure and methodology are the same thing. As usual though, they are not.  And, it's an important distinction for a handyman or anyone in a trade or busy at a project building,  making, or fixing something. 

In our situation,  a procedure is basically an outline or the steps and order of things to be done for a general project. Like, say, "Fix the roof." That is the project and there is a procedure to be established and followed by everyone working on the project to get it done properly.   

Not everyone may be involved in every step.   But by everyone knowing the procedure, they can know what "should be" going on so that they can be prepared to do their part and what and when they need to do what they need to do and what others should do or should have done to set each other up properly. 

If you're a one person crew, this is simplified but still important to keep oneself on track.

So on the "Fix the roof" project, the procedure might be 

1, clean area completely 

2, general assessment of damage(s)

3, detailed assessment of damages

4, demo damaged areas

5, cleanup

6, repair damaged areas

7, assess/inspect repairs

8, cleanup

Each of these steps may (likely) have a sub procedure of what you do for each step.

Now say for example, one of the sub procedures is to repair a section of damaged wood sheathing.  When removing the shingles, the felt, and getting to the wood sheathing there might be half a dozen ways that cutting out the bad, replacing it, and putting new felt and shingles back on, could be done.

BUT

Over time, you've learned a particular way of doing it that reduces the most common problems that could pop up as a result.  It requires that you use a particular set of tools, in a particular ways and in a particular order so that the patch is done right.

That is methodology.   It applies specifically to a task, under a particular set of conditions.   If the conditions were different,  you would use a different method to achieve the same result and achieve this part of the whole procedure.

The most important thing about procedures and methodology is communication.  If not everyone is in the loop and clearly understands, there will be unnecessary problems that will cost time, money, and maybe even damage or the need to go back and do parts or all, again.

If this is a group or team project, this becomes extremely important and more complicated.  Even if it's just one person,  it can be tricky. 

Essentially,  putting together procedure and methodology for a project is part of planning. And Proper Planning Prevents Poor Performance.

The more time and effort we put into procedure and methodology, the better prepared we are to do the project and, hopefully,  less likely to be asked "Do you know what you're doing?"