Typically most or the common smart phone has the back cover already removed. if the phone is new of course. But if the cover has been installed, locate a indent/curvature slot locate at one of the comers. Most of the time a thumb nail or paper clip can be used to pry off the cover. But be warned, the cover's design was made to retain allot of stress so come prying form all around the cover should release all the connecting point.
Once the cover has been carefully removed and if the battery has also been removed.. the sim card slots could be above or beside the battery terminals. Some smart manufacturers have actually placed the sim card mounting point under the battery and because you didn't post what phone you have.. I'm unable to clearly give a precise location of your sim card slot or even if you have the correct size.