All of the grasses and trees seen in my photos have been done this way, and it is quick to do.
You will need:
Corrugated card board (the type boxes are made from)
A hot glue gun with glue sticks
Trees, grasses or what ever you intend to base.
Paint - the colour you wan the soil to be
Step 1: cut a section of card to the size and shape you want your for your tree base. Make sure you have plenty of room for your tree and that the base will be large enough so it wont tip over once made.
cut some masking tape into small sections, and place one onto the edge of the base.
cut tabs in the piece of tape applied to the cardboard and fold them over one at a time so as to cover the sides of the cardboard. Ensure you can fold each consecutive tap slightly over the top of the previous tab.
Keep adding sections of tape to the cardboard until it has been covered on all edges, cutting tabs and folding as shown above.
Step 4: add a blob of glue from the hot glue gun to the area you want your tree to be on the base
Step 5: while the glue is still soft push your tree firmly into the glue. You may need to hold your tree in place until the glue starts to cool down and harden.
Step 8 spread white glue over the base and apply your flock for ground cover
Step 9: enjoy your new piece of scenery