i don't speak english. but I will try to explain as clear as possible

1. create square as usall

2. add stroke to the square so it looks like this

3. then click path > stroke to path

4 path > break appart you will get something similar to  

5. then delete the outter box and you get something similar to this

hope that helps

I don't speak English. but I will try to explain as clear as possible

1. create a square

2. add a stroke to the square so it looks like this

3. then click path > stroke to path

4 path > break apart and you will get something similar to 

5. then delete the outer box and you get something similar to this

hope that helps