So it looks to be just a simple snap cap, if that is the case. Then it is just a press fit until it snaps over the rim. Coolant should be half between cold and hot mark at rest.
If you are using green coolant, most likely. Then look for coolant traces around reservoir, check all hoses and connections for condition and tightness.
Find your water pump and check that there is no fluid leaks around that area. Does the fluid pool under car overnight? I'd assume it was the water pump or a failing hose.
crwdns2934105:0crwdne2934105:0
crwdns2934113:0crwdne2934113:0
crwdns2915270:0crwdne2915270:0
crwdns2889612:0crwdne2889612:0
1