1. At Admin -> Settings -> Tweaks Editor, place your custom field name in the extrawysifields tweak. If you have multiple, separate with commas.
2a. If it's a listing field and you're running WSN 9 or later, go to Admin -> Listings -> Listing Fields and use this code in the display html area when adding/editing the field: <textarea name="yourfieldname" id="yourfieldname" class="submitdescription">{LINKEDITORCONTENT[yourfieldname]}</textarea>
2b. If it's not a listing field, or you're using WSN 8 or older, use this code in the submit/edit template:
<tr>
<td class="labelscolumn"><span class="labels">Your Custom Field:</span>
<p class="center"><select name="editormode">{EDITORMODEOPS}</select></p>
</td>
<td class="optionscolumn">
<textarea name="yourfieldname" id="yourfieldname" class="submitdescription">{LINKEDITORCONTENT[yourfieldname]}</textarea>
</td>
</tr>
<td class="labelscolumn"><span class="labels">Your Custom Field:</span>
<p class="center"><select name="editormode">{EDITORMODEOPS}</select></p>
</td>
<td class="optionscolumn">
<textarea name="yourfieldname" id="yourfieldname" class="submitdescription">{LINKEDITORCONTENT[yourfieldname]}</textarea>
</td>
</tr>