It's fairly easy to draw the stereochemistry at the alpha carbon of L-amino acids, but two amino acids have side chains with their own absolute configuration, threonine and isoleucine, where now the beta carbon atoms also have to be accurately drawn.
Here is a handy way to draw the stereochemistry correctly. It relies on the use of the Newman projections at right. Starting from valine (with its achiral sidechain), draw the amino acid as part of an imaginary peptide with the amino group on the right, the carbonyl group on the left, and the alpha hydrogen atom pointing "up." The L-amino acids drawn this way have their sidechains pointing "back." Thus, the beta carbon atoms in the pictures are "behind" the circle part of the Newman projections. With valine drawn such that its methyl groups bisect the C=O and H on the left and the -N- and H on the right, with the valine sidechain H atom pointing "down," it is now a simple matter to change the right side methyl group (indicated with an arrow) to either OH (to give threonine) or to an ethyl group (to give isoleucine) with the correct side chain stereochemistry.
