even edges and blocking

Crocheting into the first and last stitch is usually what causes the edges to be either straight, or uneven.

Turning chains:  make it easier to turn the project to start the next row, and raises the yarn up to the level in the row you're about to crochet.
Turning chains for different stitches (sts):

Single crochet (sc) -  1 chain (ch):  The turning ch does not count as a stitch (st), therefore you work into the first st of the row. At the end of the next row you do not work into the turning ch, because it doesn't count as a st.

Half-double (hdc) -  2 chs:  The turning ch may or may not count as a st, therefore, check the pattern and follow the instructions.  When ch2 does not count as a st, work into the first st directly below the turning ch. On the return, at the end, skip (sk) working into this turning ch.
When the pattern says that the turning ch counts as a st, count it as the first st and therefore, sk the first st and work into the 2nd st in the row.  At the end of the return row, work into the top of this turning ch, as it counts as a st.

Double crochet (dc) - 3 chs:  More often than not, the turning ch for dc does count as the first st,  therefore sk the first st and work into the 2nd st in the row.  At the end of the return, work into the top of this turning ch, as it counts as the last st in the row.
Counting the first st as a st, crocheting into it, and skipping the turning ch at the end will slant 2 sts to the right in every single row.

When beginning with the 2nd st and forgetting to work into the turning ch at the end of the row, a st on the left edge will by dropped, then when turning and working the other direction, another st is lost, or dropped, at that edge.

These dropped sts can be picked up when not too far along in the project.  Crochet an extra st at one edge, and a few rows later work an extra st at the other edge. These gradual corrections should barely be noticeable, especially when it will have an edge or border.  An abrupt change in the pattern might be too obvious.  After so many rows of adjusting, incorporate the correct method throughout the rest of the pattern.

To block or not to block (finishing):

Pushing straight pins downward, pin the item all around onto either a double layer, or folded, damp towel.  This also helps to straighten and freshen up the item after it's completed.  Lightly spray it with water, or steam, and lightly press with an iron set on a cool temperature.  If a wool yarn was used, it might be best to pin and air-dry without pressing.  Be careful with acrylic yarns also, since pressing can flatten the yarn too much.