Creating a square crochet coaster is a simple and fun project that can be completed by beginners and advanced crocheters alike. Below is a basic pattern to get you started. You can customize it with different yarn colors and patterns once you're comfortable with the basics.

Materials Needed:
Worsted weight cotton yarn (any color)
Crochet hook (size H/5.0 mm recommended)
Scissors
Yarn needle for weaving in ends
Basic Square Coaster Pattern:
Abbreviations:
ch: chain
sc: single crochet
sl st: slip stitch
Instructions:
Foundation Chain:
Ch 16. (This will create a coaster about 4 inches square. You can adjust the number of chains to make a larger or smaller coaster.)
Row 1:
Sc in the second ch from the hook and in each ch across. (15 sc)
Ch 1, turn.
Rows 2-15:
Sc in each stitch across. (15 sc)
Ch 1, turn.
Border:
Once you've completed the last row, do not fasten off.
Ch 1, turn.
Sc evenly around all four sides of the coaster, placing 3 sc in each corner to keep the edges from curling.
Join with a sl st to the first sc of the border.
Finishing:
Fasten off and weave in any loose ends using a yarn needle.
Optional Variations:
Color Changes: For a striped coaster, change colors at the end of any row.
Textured Stitch: Instead of using sc, try using half double crochet (hdc) or double crochet (dc) for a different texture.
Edging: Add a decorative edge by using picot stitch, crab stitch, or shell stitch for the border.
Tips:
Using cotton yarn is recommended for coasters because it absorbs moisture well and is easy to wash.
Ensure your stitches are tight enough to create a dense fabric, which will help the coaster be more absorbent and durable.
Example Pattern with Color Change:
Foundation Chain:
Ch 16.
Row 1:
Sc in the second ch from the hook and in each ch across. (15 sc)
Ch 1, turn.
Rows 2-5:
Sc in each stitch across. (15 sc)
At the end of Row 5, change to a new color.
Ch 1, turn.
Rows 6-10:
Sc in each stitch across with the new color. (15 sc)
At the end of Row 10, change back to the original color.
Ch 1, turn.
Rows 11-15:
Sc in each stitch across. (15 sc)
Ch 1, turn.
Border:
Sc evenly around all four sides of the coaster, placing 3 sc in each corner.
Join with a sl st to the first sc of the border.
Finishing:
Fasten off and weave in any loose ends.

Feel free to experiment with different stitches, colors, and borders to make your coasters unique!