i assume you have an home dsl router or similar and it has a web interface. nowadays every home router has nat/pat features and that's what you need!
nat = network address translation: you set the lan ip of your server (i.e. 192.168.0.3) and that will make your router translate your public ip address into your server lan ip...in few words, your server becaome totally visible to internet.
this is the simplest solution, but it exposes your server to cracker attacks and believe me...either you are using windows or linux, unless you are a super geek, your server will be cracked in less than one hour (and you will not notice)
the best solution is pat = port address translation.
basically you have to set into your router one rule for each tcp or udp port your server have to expose to internet.
a rule needs you to specify the port number to expose to internet and the port number of your local sever -> for god's sake put the public port equal to local or you will have unuseful headaches
then you must specify also the lan ip of you local server and also the range of internet addresses allowed to connect -> put "any".
i believe the lfs standard port is 63392 set a rule for udp connection to that port, i don't know what tcp port lfs uses but you will need a rule for that also, to find it type "netstat -a" without quotes into windows command prompt (start->run->cmd)
hope it helps