Hi Mike, If you're using conventional seam tape one side will have a loose-weave mesh covered in stripes of a hot melt glue; this side adheres to the backing of the carpet. This tape requires the use of a seaming iron; DO NOT attempt this with a household iron, and NEVER let the iron come in contact with the face of the carpet. If you rent a seaming iron, it will come with a tray for the iron to sit on, which can sit on the face of the carpet, but be careful of hot glue strings or drips coming off the iron getting onto the face of the carpet.
It sounds like you're seaming a doorway, which is a bit tougher than general seaming, because your iron is trapped at each end of the seam.
Cut your tape to length to fit the area, then lay it glue-side up on the floor (presumeably on top of the pad)where it centers on the seam. If this is a doorway, you can lightly press each end of the tape down onto the tackstrip to keep it from sliding. Pre-align your carpet cut edges so they meet evenly down the length of the seam, without gaps or "puckering".
Once your iron has preheated, roll open the seam at one end, remove the iron from it's tray & place it on the tape, allowing the carpet flaps to rest on top of the iron's body, so just the handle is visible. Note that where the handle meets the body of the iron, there is a ridge which tapers to a point facing each end of the iron; this is acts like a "rudder," parting the seam, then allowing it to close. This is the point where you'll want to watch as you go along that the carpet backing is mating correctly as you work.
As the iron leaves the heated tape, use one hand behind the iron to press the carpeting into the tape, then rubbing the surface in line with the seam (not side to side).
When you reach the end, if doing a doorway, carefully hold the carpet edges open while removing the iron, (tipping it out front-to-back or back-to-front) and be sure the tape remains in place. (It may want to stick to the iron & come out with it.) Set the iron on it's tray, then roll the remaining seam together into the tape; both sides simultaneously, as a zipper would do.
Once the seam is done, allow it to dry for 20-30 minutes undisturbed before any stretching work or allowing traffic on it.
The speed of running the iron down the seam is hard to predict or explain, but basically you'll want the glue to be thoroughly melted as you go, but not allowing the iron to remain long enough to melt padding below the tape. You'll be moving much slower than if you were ironing clothing, usually progressing at a rate of around 8-12 seconds per foot. Also, you don't "press" the iron down; let it glide along the tape, only pressing forward lightly as you go.