How to Share Internet Connection?

I currently have a network setup like this:

computer A —> USB ADSL Modem —> Internet
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Linksys Router
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computer B

How do I set things up such that computer B can connect to the internet via computer A’s connection?

I tried simply right clicking on the internet connection on computer A and sharing it, but it then assigns my LAN connection a static IP address and computer A no longer becomes part of the network (or so it seems, as I cant access my router from it).