You can plant most annuals now (mid April).  Just go to your local garden center and pick out something you like.  You can select a plant for the conditions sun, shade, type of soil etc.  Have a look at the packet and you will find out how deep to plant the seeds.  Assuming there is a gap in your border (which there is)  just make some scratches or grooves to the right depth.  You can make a pattern like a circle or an X so that you can then recognise the flower seedlings from the weeds by their being in this shape.  Water the grooves and then sprinkle the seeds into them.  Cover with fine soil and wait for the flowers.  You will probably have to thin out as the seedlings develop to the distance as advised on the packet.