Start Microsoft PowerPoint and choose a new Blank Presentation. Afterwards, go to File->Save As.. to save it immediately. Remember to save your work every two minutes.

OR

Download a PowerPoint graph paper template in 1/4" scale.
(Right-click the above link and Save Link As....)

Double click on the border of the text box that reads "Click to add title." When it is selected, use the backspace (or delete) key on the keyboard to delete it.

Do the same with the "Click to add subtitle" text box.

Check the Ruler button from the View tab at the top of the screen (if it is not already in view).

Also check the Gridlines button. This adds a background grid to the screen that lines and shapes will snap to to make them lined up neatly.

Then, click the little, boxed arrow to the right of the word Show. This brings up the Grid and Guides dialog box.

Set the Spacing for 0.25 Inches (1/4 inch), which is the preferred size for making floorplans to scale. Check Display grid on screen.

To draw walls and windows:

Select the line tool from the Shapes section (inside the Home tab).

Drag a line in the document to create straight walls and windows. Keep lines straight by holding down the shift key on the keyboard while you drag a line.

Change the thickness of the line to differentiate between walls and windows. Right-click on the line and choose Format Shape. This brings up the Format Shape dialog box. Adjust the Width. Use 6 pt width line for walls and 3 pt for windows.

Remember to draw in scale, so that 1/4 inch of your line drawn in PowerPoint equals 1 foot of wall (or window) in real life. Use the top and side rulers as guides.

If you have been working only with the line tool, and you wish to fill in an area with color or texture (to create a floor carpet for example),

1. Select the Rectangle, Circle, Freeform or Scribble tool from the Shapes section.
2. Click on each corner of the area you wish to fill, forming a closed shape that will be on top of the existing area.
3. Right-click on that new shape to access the Format Shape dialog box and customize the area (change the color, texture, gradient, etc.)
   
4. Right-click on the shape and choose Bring to Front or Send to Back to move the filled area behind other objects it may be covering.

Other useful shapes (like the Arc tool, used for showing which way a door opens) can be accessed also from the Shapes section.

Arcs can be changed by adjusting the yellow diamonds after they are selected.

After drafting the walls and windows, doors and closets to scale, add dimension lines. Try to draw the dimension lines outside of the floor plan of the room. Use a very thin line width (such as 1 pt) and choose arrow heads for the line style.

Use the Shapes and Line tools to create your own furniture. Make sure your furniture is to scale and is in proportion to your floor plan. Also, make sure it is in floor plan view, which is like from the ceiling looking down.

Use the Text Box tool from the Insert tab to add text. Create a text box for the key, the scale, descriptions of furniture, and the total area of the room.

Also use text boxes in your dimension lines to show the length of the walls on the dimension lines.

Right-click on a text box to edit the text.