This cardigan is constructed from at least five different crochet patterns/motifs, a large portion of which were pieced together with the trellis stitch.  The sleeves are based on a scarf pattern, which is also used in the upper body of the cardigan.

The cardigan's buttons are also crocheted.

Following is a rough description how I put this cardigan together. I also used a simple schematic for a summer cardigan such as this below.

You may use any cardigan schematic to determine the shape and size of your cardigan - and feel free to modify.

SLEEVES

Use this scarf pattern to make the suitable length of sleeves.

Hack this scarf pattern to make the sleeves.

Make the cuffs for the sleeves using a pattern for a purse.

Hack this pattern for a purse to make the cuffs and the motifs along the front of the cardigan.

BACK

Working the hems:

Start with 1 row of Double Crochet.
Use pattern from crocheted Pillow Cover (3 rows in alternating colors).

This pillow cover pattern is used for the multicolor hemline of the cardigan.

Working the back from bottom up:

Into the row of dc, work Treble Clusters found in the Scarf Pattern.
Then continue with the Scarf Pattern’s trellis stitch, then Treble Clusters.
A lace edging for a capelet pattern is then used.

Use the edge from this capelet pattern to make the upper back section of the cardigan.

Make 7 rows of alternating colors with Parquet Stitch as shown here.
This is now where the armhole begins.
Proceed with Trellis Stitch for suitable length, decreasing to shape the armholes.

FRONT RIGHT SIDE

Working the hems:

Start with 1 row of Double Crochet.
Use pattern from crocheted Pillow Cover (3 rows in alternating colors) (see pattern above).

Working the front from bottom up:

Continue with Trellis Stitch and Treble Clusters found in the Scarf Pattern.
Then use the border of this pattern for a capelet.
Now, use this pattern:

This openwork pattern is used for the upper section of the front of the cardigan.

FRONT LEFT SIDE

Same as Front Right Side but done in reverse.

ASSEMBLY

Use sc to assemble.

NECKLINE

Use this pattern for the neck area.

The neckline of the cardigan is based on this pattern.

FRONT SEAM AND BUTTONS

Make suitable length of motif of pattern from the lace purse as shown above. Sew along front of cardigan opening.
Crochet small buttons and sew along other side, align with small gap between motifs which serve as button holes.

Weave in all ends.

The back section of the cardigan. Rows of Parquet Stitch in different colors was used to separate the lower and upper sections.