Probably best to look around on youtube, but if you can make sense of the following, you might be able to sort it out:
1 - Create folder in your FM graphics folder called 'Kits'. This can be found normally throught My Documents > Sports Interactive > Football Manager XX
2 - Copy and paste your kits into that new folder
3 - Open Notepad and copy the following text into and then save this text file as config.xml in the same folder as your kit. You will need to replace the names of the images with whatever you are calling them i.e. Home or Away and replace the team ID with whatever the ID number for your team is. You may need to switch on this option within the settings menu in game to see the ID number:
<record>
<!-- resource manager options -->
<!-- don't preload anything in this folder -->
<boolean id="preload" value="false"/>
<!-- turn off auto mapping -->
<boolean id="amap" value="false"/>
<!-- picture mappings -->
<!-- the following XML maps pictures inside this folder into other positions
in the resource system, which allows this folder to be dropped into any
place in the graphics folder and still have the game pick up the graphics
files from the correct places
-->
<list id="maps">
<record from="PICTURE NAME" to="graphics/pictures/team/CLUB ID NUMBER/kits/home"/>
<record from="PICTURE NAME" to="graphics/pictures/team/CLUB ID NUMBER/kits/away"/>
</list>
</record>
4 - There's more information in this guide here that explains the xml side of things if the above doesn't make sense.
https://www.fmscout.com/a-how-to-make-xml-config-files.html