I am a Community Health Nurse who formerly did EPSDT testing, which included the DDST. As some respondents have indicated, if you are going to be doing DDST as part of a job role, you should be certified to do it. That means you should attend a one or two day training and submit a video of yourself doing the DDST with a child to a trainer for "grading" or whatever the training process is these days.
Like any standardized screen, the training is necessary so that the examiner knows which tasks to present, how to present the tasks, knows what to allow and what to disallow. You also need to know how to interpet your findings! I've seen well administered Denver's and not-so-well administered DDST's. The sensitivities and specificities of the Denver occur when it is presented and interpreted in a standardized fashion.
That said, it sounds like you're a student having a standard student experience. There is a manual that goes with the kit and explains how to present it, item by item. If you don't have this manual, any "testing" you do will not be worth the time it takes to perform it. (the testing grid is good but not a complete instruction to administering the DDST.) So, if you are a student having a student experience do the best you can, but realize you have not really evaluated anything.
RE: the above comments. My husband is a doc in FP residency and they work relentlessly on developmental delay case finding. Some docs will be more oriented to it than others. Additionally, when I worked with severely affected, special needs kids, many of my kids had rare disease entities OR atypical presentations of disabilities and they constantly were "teachers" to their pediatricians and FP's alike. I do not necessarily expect FP's to dx autism and it's variants or other developmental delay problems BUT I expect them to have an index of suspicion for them and to know their referral resources.
Finally, RE: the nurse mom who's kids couldn't stack blocks (but showed the ability to learn the task during the appointment time). In my experience, this was not uncommon--when the child simply had not been previously exposed to the task, they often couldn't perform it. If you have a continuous relationship with the kid (you work in a doc's office), you have the luxury of telling mom they need to have the kid stack blocks at home and retest in several weeks BUT if a child has a cluster of "Fails", has a high risk history (premie, LBW, serious illnesses) AND doesn't seem to be just having a bad day (have you ever tried to do a DDST on a toddler at their typical nap time?), then you should seriously consider referring to your early intervention team. REMEMBER, the DDST under-refers and really doesn't do a great job of testing speech or hearing. In general, nurses, the more DDST you do, the better quality product you generate so long as you stick to the testing protocol.
Good luck, Ms Purp