If you use straight Portland bags, you will need to add an aggregate and sand. The mix ratio will be 3-2-1. For instance if you are mixing in a wheel barrow say, You will shovel in sand first (2), then Portland (1). Dry mix this together. I'll then put water to it, and mix in the aggregate or stone (3) as I go. If you have a mixer, toss it all in and have at it.
And any solid aggregate will work. I use 2B stone, but pea gravel will even work. I'm assuming your application will not be supporting the Taj Mahal. Oh, and, be sure to use concrete sand and not the sifted masonry sand (fine).
Thanks folks, great suggestions. I decided to just go with some concrete ready-mix. It's relatively cheap and not worth taking the chance. I just need to coem up with a project now to eat up the mortar.