Floor cloths seem to be quite the fashion these days, and they are very easy to make. There are a lot of ways of creating a painted floor cloth, from the time-consuming traditional method using linseed oil to much quicker procedures using modern products. Here are directions for four different methods: 
Old-fashioned classic floor cloth with linseed oil 
A quicker way with an old-fashioned floor cloth 
A floor cloth using modern materials.
A modern floor cloth--- how to cheat if you're not an artist!

1. Old-fashioned classic floor cloth with linseed oil. 
This method takes more time (mostly in drying), but if you want an authentic period piece, this is the way to do it. (Linseed oil is made by pressing oil from the flax seed, and was a common product for hundreds of years since flax is the plant that produces linen for cloth.) 

Use good quality canvas, duck or denim, and wash to remove any sizing. The piece should be about six inches larger in both dimensions than the finished floor cloth to allow for trimming and hemming. Stretch the canvas, using staples or tacks on a wooden frame. Saturate the canvas with linseed oil, spreading the oil gently with a paint brush so that it does not puddle or drip. Make sure the oil penetrates through to the back side. You may need to apply oil to the back as a separate step. 

Hang the canvas outside, preferably in warm, dry weather. It will take several days to a week to dry completely depending on the temperature and humidity. The linseed oil will stiffen the cloth as it dries and create a hard surface to the touch. 

When dry, stencil or paint a design on the canvas using any of the following: natural pigments dissolved in linseed oil, artist's oil paints thinned so that they aren't lumpy, oil-based exterior house paint, spray paint (non-acrylic) or oil-based hobby paints. After the paints are thoroughly dry (allow 3 days, except for quick drying spray paints), seal the surface with a clear varnish. Apply at least 3 coats of varnish, allowing each coat to dry completely. 

Cut the canvas from the frame with a razor knife. The edges can be left as is, or folded under for a hem, and glued with contact cement. Hemming wasn't often a worry with the old cloths, and varnish can be applied around the edges to seal any fiber ends. These floor cloths will stand up to foot traffic, but if you notice a wear pattern, re-varnishing is in order. 

2. A quicker way with an old-fashioned floor cloth. 
Use heavy canvas, denim or duck, and pre-wash to remove any sizing. Stretch the canvas as above on a wooden frame. Paint the front side heavily with old-fashioned shellac, spreading it evenly over the front side of the canvas. You must work quickly because shellac dries fast. When the front side is dry, turn the canvas over and repeat the process with the back side. 

Once the shellac is completely dry, you can use almost any paint, oil-based or acrylics, to paint or stencil your design. Use brighter colors than you want in the finished floor cloth since the finishing layers of shellac will dull and slightly yellow the painting-which actually gives it a nice mellow look. 

When the paints are thoroughly dry, finish the floor cloth with at least two more coats of shellac. When the shellac is dry, cut the canvas from the frame with a razor knife. As above, hemming is optional. 

3. A floor cloth using modern materials.
Pre-wash the canvas, duck or denim, and stretch on a wooden frame as above. If you want a white background for your painting, coat both sides of the canvas with artist's gesso (available at art supply stores). If you want the natural color of the fabric to show through coat both sides of the canvas with a product called "Polycrylic" made by Minwax and available at paint and hardware stores. 

Allow the base coating to dry thoroughly before painting. Use acrylic paints to paint or stencil your design. When dry apply 2 coats of the Minwax Polycrylic, letting each coat dry thoroughly. Cut the canvas from the frame with a razor knife. 

With the acrylic based products a hem works best since the base coats do not penetrate the fibers well. Fold over an inch or more around the edge, mitering the corners and use contact cement to hold the hem in place. (Remember that contact cement "grabs" immediately so fold over the hem carefully.) 

(On a personal note, the Minwax Polycrylic is incredible stuff for floors and floor cloths and has such a hard finish that it even resists pet toenails. No, they aren't paying me to say so, and I've used it in a number of different projects and been uniformly delighted with its performance. I really recommend it.)

4. A modern floor cloth --- how to cheat if you're not an artist! 
Use heavy canvas, duck or denim, but do not pre-wash it. Iron it so that there are no wrinkles. Apply iron-on fusible webbing (the kind that comes by the yard and has a peel-off backing) to the front of the canvas. Then select fabrics with patterns or designs that you like, and cut them to the size and shape for quilt patterns, a framed picture or whatever strikes your fancy. Use calicos, chintzes or picture panels for your design. All of the pieces should be about the same weight of fabric. (Don't mix heavy cottons and light cottons on the same floor cloth.) 

Make sure that the whole surface of the fusible webbing is covered. Try to match the edges of the pieces, but a small overlap (not more than an eighth of an inch is fine.) Iron the pieces in place, making sure that they are all secured. 

Cover a table (or the floor) with plastic sheeting, and lay the floor cloth face up. Paint the front side with at least 2 coats of Minwax Polycrylic, letting each coat dry. Then paint the back side with 1 or 2 coats of the Polycrylic. Trim up the edges so they are even, and you're all done. 

(How to REALLY CHEAT. If you can find a heavy canvas, duck or denim that is already printed with a design you like, just follow the directions in the last paragraph for an "instant" floor cloth!)