I am just started using silverstripe. Now I'm trying to install UserForms module. I successfully installed it by extracting the downloaded file, renaming it to userforms then placing it to the root folder, then build the database by dev/build.
If you're using the default BlackCandy-theme it's pretty easy: all is said and done...
If you're using your own (custom) template, do this:
1) edit this file (themes/YourTemplateName/templates/Layout/Page.ss), then add $Form (most of the time you will add it below $Content, but that's up to you ;-) )
6) edit (if you want to) the default e-mailtemplates in userforms/templates/email
7) _config.php is referring to mysite/_config.php
Hope this helps. Working trough the tutorials will help you understand the SilverStripe (templating)structure (http://doc.silverstripe.org/doku.php?id=tutorials - at least tutorial 1 and 2) . They're imo pretty clear compared to many other CMS's. It helped me alot in my own search for the $Form-tag and many other tags and functions.