Note that these code placements are not set in stone, and are merely suggestions.

Brooklyn Theme

/customers/account.liquid - The snippet for this could be placed between </header> and <div class="grid"> (line 16 in the below screenshot), or between the two </div>s above the
{% if paginate.pages >1 %}

/customers/order.liquid - The snippet for this could be placed between
</header>, and <div class="grid"> (line 7 in the below screenshot),  or between the final </p> and the </div> (line 151 in the below screenshot).

Minimal Theme

/customers/account.liquid - The snippet for this could be placed above <div class="grid"> (line 11 in the below screenshot) if you'd like it to be close to the top of the page:

You can also place the code above <div class="grid__item one-third medium-down--one-whole"> if you'd like it below the orders, but below the addresses, or below the closing </div> a few lines later if you'd like it below the addresses as well.

Narrative Theme

/customers/account.liquid - The snippet can be pasted between the </header>, and <div class="grid grid--rows"> (line 9 in the screenshot below)

/customers/order.liquid - The snippet can be pasted between the </header>, and <div class="grid"> (Line 9 in the screenshot below)

Symmetry theme

/customers/account.liquid - The snippet can be placed between </div>, and <div id="customer_sidebar"> (Line 5 in the screenshot below)

/customers/order.liquid - The code can be placed between </div>, and {% if order.cancelled %} (line 6 in the screenshot below)

Debut Theme

/customers/account.liquid - The snippet can be pasted between </div> and <div class="grid myaccount"> (line 6 in the screenshot below)

/customers/order.liquid - The snippet can be pasted between </div> and <div class="grid myaccount"> (line 6 in the screenshot below)