How do you draw out a Lewis dot structure, BF3 for example

You use the element symbol and draw the valence electrons. The valence electrons is the number of electrons on the outer shell. An easy way to know how many valence electrons there are, you can look at a periodic table and see what group the element is in. Group 1 has 1 valance electron. Group 2 has 2 valence electrons, and so on ... so in the example you gave me, since there is 3 fluorines, and 1 boron, you want to arrange the 3 fluorines around the 1 boron so the electrons can bond with each other.