There is a tendency to assume that plants are difficult (not true) or fiddly (well, a bit) to make. As a consequence, most battles are fought out over grassy parkland as bare as football pitches or, alternatively, deserts. But it is not difficult to make good looking plants.
I use three approaches: ready-made plants, home-made plastics, and home-made paper.
Corrugated cardboard, plywood or MDF for bases
Thin see-through or translucent plastic - I use the plastic from bottles of milk sold in supermarkets.
Using ready made plants doesn't require much effort at all. Take a trip to your local aquarium or pet store and check out all the different plastic plants available. They can be quite expensive but you can get cheap ones and repaint them quite effectively.
Making paper or plastic plants is easy and cheap. The method is basically the same for each.
For paper plants begin by cutting a piece of white paper twice as high as the plant you want and about 6 inches long. Fold it in half and glue with PVA glue. This paper and PVA sandwich will set to a plastic-like stiffness.
For both paper and plastic plants mark out the leaf shape that you want.
Cut the leaf pattern out of the edge of the paper. If you end up with a part leaf at the end just snip it off back to the last full leaf. For plastic plants cut the shape out of plastic in just the same way.
Paint PVA glue along the bottom edge of the paper and roll it up. For plastic plants don't glue it but you will need to stick a pin through the bottom to keep it rolled up.
For paper plants you can now get hold of the centre leaves and pull them upward, giving the plant some height. You can make really tall plants this way. For tall plants, cut your leaf patterns smaller for the leaves that will be near the top (those in the middle of the roll). If you have pulled the plant up you may need to dab extra PVA glue in to fix it. Bend the leaves outward a bit to give a more natural appearance.
Paint your plants up in, well, a planty fashion. For plastic plants don't undercoat - instead, use a weak green like Citadel Scorpion Green. This will leave your leaves with a slightly translucent appearence which is quite pleasing. This is a paper plant painted in Citadel Emerald Green with Goblin Green stripes.
An alternative method for is to cut the leaves out of plastic or paper individually and then stick them into a body of air-drying clay. The large grassy plants amongst these rocks were made in this way.