Those are very nice and a great idea. I recently bought a couple of alphabets at Michael's (near their stepping stone kits) and hope I can be as creative.

Hi Fleur

If you using the letters to put words in concrete, consider using a vinyl concrete instead of a regular mix of portland cement and sand. I have tried both and the vinyl concrete mix seems to work best at holding the letter shapes. As with everything there is a learning curve and I am rounding the bend on that curve. :-)
-Too wet of a mix the letters don't hold well
-Too dry of a mix and you can't push the letters in well
-Use a trowel to level the surface of the object before pressing in the letters
-Don't rock the letter while pushing it in or it makes the letter less distinct, just use both hands with even pressure and push straight down
-Leave the first letter in the concrete while you are pushing in the second letter. It helps to hold the shape of the first. If you can, leave all letters in briefly before pulling the letters straight up and out.
-Very small letters with inside counters i.e.-the center of the "o" or the "a" make take a couple of tries to get just right
-Best of all... if you don't like how it looks just use the trowel, smooth it over and try again!!

Happy lettering!
Nancy

It almost looks like the word brick is copper on the planter!