For Portobello's, in most climate zones growing indoors under a controlled environment is optimal.
The best way for beginners to grow Portobello mushrooms is from a kit. The kits retail for anywhere from $40- 50 . You open the box - read the instructions - keep it moist , and in about a month - you'll probably have about $25- $30 worth of mushrooms for your $50 investment. However you will have had a learning experience.
If you're a gardener, who enjoys reaping the fruits of your labor and enjoys an occasional new project to experiment with, you should consider Portobello mushroom spores . Spores are basically mushroom seeds , they are how fungi - which is what a mushroom is - reproduces.
Once you have the spores you'll need a growing medium or planting bed to grow the mushrooms in. You'll also need a container or tray . Flower pots generally won't cut it - the container should be deep enough to hold your growing medium - [slightly rotted compost and some peat is fine] and a dark location where the temperature will remain a steady 60 - 70 degree F
Fill your tray with the partially decayed compost, leaving 1 1/2 inches of headspace at the top of the container. For every 6 Sq. feet of planting surface you'll need about 1 1/2 cups of dried spores. Sprinkle the spores onto the compost and slightly compact them in by pressing firmly on the surface - I didn't say to squash the little buggers _ I just said press firmly. Sprinkle some peat moss over the surface after you've laid your spores, about 1 1/2 inches of damp peat is sufficient.
Some damp newspaper over the peat for the first week is good also- I don't like keeping the newspaper there too long as it invites mildew and competing native fungi - which are generally toxic.