In addition to decal paper, you can also use white sticker paper.
Just make the sign in the size, color and font style you want on your computer using Word or a similar program and then print it out on the sticker paper. Cut to size and just stick it on or over whatever you want.
Tip #1- Make multiple signs in different sizes, colors and fonts on the same page, so you can test fit and choose which ever you like best.
Tip#2 - Print out on plain paper first to test fit and then a final print on the sticker paper so as not to unnecessarily waste the sticker paper.
Tip #3 - When you finally decide on the final size, color and font style you want, print out 3 or 4 of the same signs on one sticker sheet in case you cut the first one wrong, mishandle or make a mistake, etc. and you'll have more readily available signs on the same sheet.
Below is a sticker sign I made for a small garden.