the easiest way to go about it is,
draw out each space, from above, and a side view,
for each space you need plants that will create, structure, colour and foliage,
you need for those categories to be represented each season, in each space,
do lots of drawing, just put in shapes and colours, when you have something that pleases you, find plants that match design,
walk around your immediate neighbourhood, every couple of weeks, note what looks good in other peoples gardens, it will properly do well in yours,
plant in odd numbers 1,3,5,7 and repeat plants in groups around the garden it takes the eye around,
and think of the cactus as structure for that particular space.
there are lots of brilliant gardening forums around so ask about individual plants on there just one of many