The next step is to level the gravel as accurately as possible so the blocks line up evenly. This will be much easier to do if you use crushed gravel in Step 1 to cover the black plastic. Level the gravel in both directions -- width and length. It helps to have a long, straight 2x4 to use in conjunction with the level. Don’t rush this step or the blocks will not fit neatly together.
Also note, you can make raised beds any size you want as long as they are even increments of your blocks. But a 3’ interior width is a good size that keeps plants within easy reach.
Now you can start setting the first course of blocks. Start in a corner and use the square to make a 90 degree corner between blocks. Set the blocks tight against each other. Use the 2x4 to keep the blocks in a straight line. Work your way around keeping the blocks straight and level and hopefully the last block will fit perfectly. If it doesn’t, then the sides aren’t square and you’ll have to adjust the blocks slightly until they line up.