In MultiMC/Prism, click on your b1.7.3 instance and click "Folder" on the right side menu. Enter the .minecraft folder, delete the contents of the resources folder there, and replace them with the files from the extracted archive.
Can't connect
This includes timeout errors, "An existing connection was forcibly closed by the remove host", and similar. First, make sure you're using either Prism Launcher or MultiMC. The official launcher does not handle old versions as well.
When you made your instance, make sure you selected b1.7.3 and NOT 1.7.3. This requires toggling the "Betas" checkbox seen below.
Finally, try connecting to the website - if you can't visit that, there is a chance your ISP may be blocking our IP address. DM me and I'll help.
I just found out how to enable fixes for Minecraft skins on b1.7.3, you just need to add the following to Java arguments of your MultiMC/Prism Launcher instance:
This seems really neat. There was always something about the simplicity of older versions that gave me joy. Applied for whitelist, hope to join you all!