Garter Stitch: Duplicate Stitch Method
Reference for the below knit kits:

Garter Stitch is created by knitting every row if you are knitting a flat item (scarf or baby blanket kits) or alternating knitting 1 row, then purling 1 row when knitting in the round (cowl kit).

Garter Stitch creates rows of alternating purl bumps that resemble a frown and a smile.  The frowns are slightly above the smiles.

With Duplicate Stitch Method, we are weaving along the path of the actual stitches in a row.  Begin, by inserting your needle up and through the nearest smile.

Then, insert the needle down and through through the nearest adjacent smile.

Underneath the smile you just came through, is a flat upside down "V".  Bring your needle parallel and underneath the upside down "V'.  Thread the end through.

Thread the needle up and through the above smile, which you recently came down through.

Repeat this sequence a few more times.  Bring your needle down and through the adjacent smile.

Then, underneath the upside down "V" below

Then, back up the smile above that you recently came down through.

After completing the sequence a few more times, trim the end leaving about 1/4 of an inch.  If you stretch your fabric a bit, the 1/4 inch end should disappear into the fabric.