As part of writing legibility, second to recognizable letters, spacing between words is very important. If there is not a noticeable space between words that is double to triple the space between letters, then all of the words run together to make it unreadable.
In the early years, kids are often taught to finger space between words. For kinder kids, they use two spaces between words because they are writing bigger at that age and need more space. As the kids progress, they start using one finger between words. Finger spacing is great because kids tend to have their fingers with them wherever they go.
Some kids have trouble remembering to finger space, and as they get into higher grades, the teacher is no longer reminding the class to do it, so they may need a more physical reminder to space between words.
There are several options to try when using a physical spacing cue. The key is having an abject to place on the paper in between words as you are writing them. Popsicle sticks are easy to come by and easy to replace when (not if) they are lost. I have the kids decorate their own popsicle stick to make it more motivating to use. Other objects you could use as a physical space reminder are a penny, tiny toy, spacer tool, rubber band, paper clip, eraser, etc. There are some commercial spacers on the market as well such as the space man, finger spacer, and star spacer.
The star spacer was created by an OT, and is a little different than the others as it is a see through plastic with a cut out space to write in. There is a star on the left side which is supposed to be placed between the words. I have a student who really lides using the star spacer and his writing is a bit more legible when he uses it because it also makes him slow down and be a little it more careful with his writing. Hopefully the spacing and the carefulness will become more of a habit so that his writing will be more legible all of the time.
Another technique that works for some kids is adapted paper. Some people make their own, and the commercially available version is call redi-space paper. Essentially, the paper has divided lines for each letter, and you leae an extra space between words. This paper works well for some kids.
Finally some kids need to have word boxes, where an adult draws a box with a pen or highlighter in the space that they need to write their word. This is the most intense of word spacing.