You have to create a FeatureType before you can create a Layer (i.e. publish it). If you have already created a test layer then fetch the feature type of that one (like http://geoserver.ianturton.com/rest/workspaces/topp/datastores/bgs/featuretypes/test_uk_625k_mapped_feature.xml) but you should be able to delete quite a lot of that and leave it for GeoServer to fill in.
<featureType>
<name>test_uk_625k_mapped_feature</name>
<nativeName>test_uk_625k_mapped_feature</nativeName>
<namespace>
<name>topp</name>
<atom:link xmlns:atom="http://www.w3.org/2005/Atom" rel="alternate" href="http://geoserver.ianturton.com/rest/namespaces/topp.xml" type="application/xml"/>
</namespace>
<title>uk_625k_mapped_feature</title>
<description>net.opengis.wfs20.impl.AbstractTypeImpl@1f89241 (value: Just for testing, lang: <unset>)</description>
<keywords>
<string>uk_625k_mapped_feature</string>
<string>features</string>
</keywords>
<nativeCRS class="projected">PROJCS["OSGB 1936 / British National Grid",
GEOGCS["OSGB 1936",
DATUM["OSGB 1936",
SPHEROID["Airy 1830", 6377563.396, 299.3249646, AUTHORITY["EPSG","7001"]],
TOWGS84[446.448, -125.157, 542.06, 0.15, 0.247, 0.842, -20.489],
AUTHORITY["EPSG","6277"]],
PRIMEM["Greenwich", 0.0, AUTHORITY["EPSG","8901"]],
UNIT["degree", 0.017453292519943295],
AXIS["Geodetic longitude", EAST],
AXIS["Geodetic latitude", NORTH],
AUTHORITY["EPSG","4277"]],
PROJECTION["Transverse_Mercator", AUTHORITY["EPSG","9807"]],
PARAMETER["central_meridian", -2.0],
PARAMETER["latitude_of_origin", 49.0],
PARAMETER["scale_factor", 0.9996012717],
PARAMETER["false_easting", 400000.0],
PARAMETER["false_northing", -100000.0],
UNIT["m", 1.0],
AXIS["Easting", EAST],
AXIS["Northing", NORTH],
AUTHORITY["EPSG","27700"]]</nativeCRS>
<srs>EPSG:27700</srs>
<nativeBoundingBox>
<minx>-132576.78915711527</minx>
<maxx>743466.6249987332</maxx>
<miny>-15669.960592884949</miny>
<maxy>1248847.1762802668</maxy>
<crs class="projected">EPSG:27700</crs>
</nativeBoundingBox>
<latLonBoundingBox>
<minx>-11.801005644638966</minx>
<maxx>4.349696858682689</maxx>
<miny>49.5245056714837</miny>
<maxy>61.11839723246307</maxy>
<crs>GEOGCS["WGS84(DD)",
DATUM["WGS84",
SPHEROID["WGS84", 6378137.0, 298.257223563]],
PRIMEM["Greenwich", 0.0],
UNIT["degree", 0.017453292519943295],
AXIS["Geodetic longitude", EAST],
AXIS["Geodetic latitude", NORTH]]</crs>
</latLonBoundingBox>
<projectionPolicy>FORCE_DECLARED</projectionPolicy>
<enabled>true</enabled>
<store class="dataStore">
<name>bgs</name>
<atom:link xmlns:atom="http://www.w3.org/2005/Atom" rel="alternate" href="http://geoserver.ianturton.com/rest/workspaces/topp/datastores/bgs.xml" type="application/xml"/>
</store>
<maxFeatures>0</maxFeatures>
<numDecimals>0</numDecimals>
<overridingServiceSRS>false</overridingServiceSRS>
<skipNumberMatched>false</skipNumberMatched>
<circularArcPresent>false</circularArcPresent>
<attributes>
<attribute>
<name>uuid</name>
<minOccurs>1</minOccurs>
<maxOccurs>1</maxOccurs>
<nillable>true</nillable>
<binding>java.lang.String</binding>
</attribute>
<attribute>
<name>lex</name>
<minOccurs>1</minOccurs>
<maxOccurs>1</maxOccurs>
<nillable>true</nillable>
<binding>java.lang.String</binding>
</attribute>
<attribute>
<name>rcs</name>
<minOccurs>1</minOccurs>
<maxOccurs>1</maxOccurs>
<nillable>true</nillable>
<binding>java.lang.String</binding>
</attribute>
<attribute>
<name>lex_rcs</name>
<minOccurs>1</minOccurs>
<maxOccurs>1</maxOccurs>
<nillable>true</nillable>
<binding>java.lang.String</binding>
</attribute>
<attribute>
<name>the_geom</name>
<minOccurs>0</minOccurs>
<maxOccurs>1</maxOccurs>
<nillable>true</nillable>
<binding>com.vividsolutions.jts.geom.Geometry</binding>
</attribute>
</attributes>
</featureType>
Experiment and when it works update the documentation for the next person along.