How can I make my own grass tufts for miniatures?
You can use a cheap paint brush.

Preparation

Use a flat, broad brush used for home decorations or an old brush you no longer use for crafting. Stiff bristles with a light color are preferable.

Seperate a small amount of bristles to one side. You can permanently bend them by applying some force or you can cover the remaining bristles with masking tape. Apply paint to the selected bristles and let it dry completely if desired.

Application

Put a drop of glue onto a flat surface (not your terrain). Grab your seperated bristles with your non-dominant hand and don't let go until they are glued to the terrain. Cut the bristles to the desired length with a pair of scisors. You should end up with a staight cut. Dab the cut edge of the bristles into your prepared glue and place it onto your terrain. Hold it a few seconds to allow the glue to set. As long as the glue is somewhat wet, you can change the shape of the tuft using a toothpick.