To plant seedlings in your straw bale, simply take a sharp trowel (I am particularly fond of Soil Scoops , and stick it down into your straw bale, wiggling it back and forth to make room for your seedling. As usual make sure to plant seedlings no deeper than they sit in their nursery pot. Also, try to place taller plants in the back of your garden, so that when they grow, they won’t shade the smaller plants.
You can also stick plants in the sides of the bales.
If you are planting seeds in your straw bales, just do it like you normally would, following the directions on the seed packet.
To get a head start on the season, you can start your seeds indoors. Here are some links for seed starting information.