Microsoft does provide some documentation for deploying SSTP VPN in Windows Server 2008 but it is a bit different than newer versions of Windows Server.
https://technet.microsoft.com/en-us/library/cc731352(v=ws.10).aspx#Configuring%20VPN1
The part that you really need to know is:
Install Routing and Remote Access
Configure VPN1 with Routing and Remote Access to function as a VPN server.
Install VPN and certificate services roles
- On VPN1, in the Initial Configuration Tasks window, under Customize
This Server, click Add roles.
- In the Add Roles Wizard dialog box, in Before You Begin, click Next.
- Select the Network Policy and Access Services check box, click Next,
and then click Next again.
- In the Select Role Services dialog box, under Role services, select
the Routing and Remote Access Services check box.
- Click Next, and then click Install.
- In the Installation Results dialog box, click Close.
Configure Routing and Remote Access
Configure VPN1 to be a VPN server providing remote access for Internet-based VPN clients.
Configure VPN1 to be a VPN server
- On VPN1, click Start, point to Administrative Tools, and then click Routing and Remote Access.
In the Routing and Remote Access console tree, right-click VPN1, and
then click Configure and Enable Routing and Remote Access.
In the Welcome to the Routing and Remote Access Server Setup Wizard
page, click Next.
In the Configuration page, click Next to accept the default setting
of Remote access (dial-up or VPN).
- In the Remote Access page, click VPN, and then click Next.
- In the VPN Connection page, under Network interfaces, click Public.
This is the interface that will connect VPN1 to the Internet.
- Click Enable security on the selected interface by setting up static
packet filters to clear this setting, and then click Next.
Normally, you would leave security enabled on the public interface. For the purposes of testing lab connectivity, you should
disable it.
Click From a specified range of addresses, and then click Next.
Click New, type 192.168.0.200 for the Start IP address, type 192.168.0.210 for the End IP address, click OK, and then click Next.
Click Next to accept the default setting, which means VPN1 will not
work with a RADIUS server. In this scenario, Routing and Remote
Access Server will use Windows Authentication.
In the Completing the Routing and Remote Access Server Setup Wizard
page, click Finish.
If the dialog box that describes the need to add this computer to
the remote access server list appears, click OK.
In the dialog box that describes the need to configure the DHCP
Relay Agent, click OK.
Close the Routing and Remote Access snap-in.
You may want to overview the whole part for Configuring VPN1 for the certificates part so you can make sure you are adding your certificates properly.
Hope this helps!