Q: timing

hii good day I live in the Caribbean in the virgin islands we do not have any Volvo deals down here. the problem im haven is timing i have a 2000 s40 volvo and i blow the head gasket but that engine was replace. the head gasket was blown because the fan was not working properly only when the a/c was on the fan works but when the a/c off the fan was not working but we found that out to late the engine was already damaged but we fix the fan and change the engine it drive very good for a while but then it started to behave like is about to over heat the temperature keeps goin up so i carry it by my friend to check it then he told me it had a lot of build up in the system very thick. so he flush it then it drive for 2 days good then it just will not start. so i buy a new head gasket change the cylinder head now me and him put it back up but just cant get it start good. i would like to know how to time it step by step instructions also does it have to be (top dead center)