The first step was to choose pictures from Bella's many coloring books that fit the theme. Valentine's day is easy because you can add hearts to anything, but I tried to choose pages that seemed lovey or that evoked Bella's personality (and that didn't contain a scribble in some can't-work-around-it spot).
Then I allowed Bella to choose from the selected pages. We chose one page per teacher for a flat card, though you could do multiple pages for a folded card or larger sized piece of paper. After choosing her pages, Bella carefully colored each page using "cweative-tivity" to make the pages special.
After she finished the first page, I cut out the main image while Bella moved on to the next page. We spread this part out over a couple days so coloring didn't lose its fun. After the image was cut, Bella applied glue (using a stick) and mounted the image on a piece of construction or scrapbook paper (or any nice paper you have, you know, that doesn't have scribbles on it).
The final step was to write a message on the page and sign the card. I wrote the message (Happy Valentine's Day! in this case) and Bella signed her name (anywhere she wanted and any way she wanted). Obviously, you could go further and really fancy these up, but the ones we gave out really felt like Bella's Valentines -- and you don't want to lose that effect.
The best part is that I didn't have to sit with Bella and pull out all the stamps and stickers and paints and glitter. I didn't spend any extra money and I didn't stress out about perfection (by which I mean that I focused furiously on deadlines while she colored).
Her teachers were beaming when they received their special cards. It's a win-win-win.