Becoming a bit obsessed with a map to keep track of what's planted where, I took to my trusty Mac to find software to help.
First let me recommend a vector draw program is much easier to work with that a paint program. A paint program allows the user to sketch using the mouse or trackpad. If you have a very steady hand and lots of patience I supposed you could make it work, but the draw program is much more forgiving. There is a learning curve, but a few hours of fiddling and you'll have what you need. Rectangles and ovals are snap. irregular shapes require laying down a series of points around the edge.
I captured a picture using the Google map satellite shot of my property (using Snapz Pro). In a draw program you create layers. This became my base layer. I then created a layer for outlining beds. I zoomed in and combined this with measured sketches to draw the basic shapes, Then by clicking on the points I dragged out handles (called bezier tools) that allow fine tuning of curves.
If you have a few hundred to give to the folks at Adobe, Illustrator is the gold standard. But after trying a couple of options I settled on iDraw. It has everything I need (and more) and for $25-$30 it is a bargain. For another $8-$10 there is an iPad version.