Treble crochet
US treble (tr) is UK double treble (dtr). Begin with two yarn-overs.
Counted practice swatch
Chain 18. Start in the fifth chain: 14 worked trebles plus the four-chain turning height counted as stitch 1 = 15 stitches.
Work the repeat
Yarn over twice, insert, pull up a loop (4 loops). Yarn over and draw through two loops three times. Later rows: ch4 counts as first tr; skip the first stitch, tr across and finish in the top of the prior turning chain.
Read the fabric
These tall posts need relaxed turning chains. Working the first stitch as well as the counted ch4 would add a stitch at the edge.