Sometimes it is required to make calls to a Custom REST API to perform certain operations, however it can be highly confusing to use APIs when one is new to it. Below are the steps to make a call using sample code.
How-To:
1. Login as an Administrator, and navigate to the IDE. To access it, either click the small tab at the top center of the page and select IDE as shown, or via URL (https://<yourportalurlhere.com>/sys/ide/index)


2. If you don’t already have an existing controller to run your desired call, create one via File > New Controller. If you have an existing controller you wish to modify, click the # tab on the left side and double-click the controller in the list to open it.


3. Enter your desired code and click Save.

Below is the pictured sample code for your own use:
public class KBArticleTestController : AspxController
{
public override ActionResponse Index()
{
var binding = ForceUtility.GetBinding("Force");
string uri = binding.Url.Substring(0, binding.Url.ToLower().IndexOf("/services/"));
string restQuery = string.Format("{0}/services/apexrest/<RestApiMethod and Parameters>", uri);
HttpRequestInfo request = new HttpRequestInfo {
Url = restQuery,
ContentType = MimeContentTypes.Json,
Accept = MimeContentTypes.Json,
};
request.Headers.Add("Authorization", "Bearer " + binding.SessionHeaderValue.sessionId);
var result = HttpHelper.HttpGet(request).GetResponseContent();
return View();
}
}