brick onto concrete slab

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I plan to cement several bricks onto a concrete slab. What do I need to know, what do I need to be careful of?
Do I need to clean that concrete slab first? It's been there for 15 years. What do I clean it with? It's not painted.

I think I'll buy a bucket of ready mixed concrete to do the job.

I had a steel security door installed. Before, one opened the storm door and stepped from the kitchen over the threshold, and down a few inches to the slab at ground level.
The security door was added to the trim of the existing door, and there is a large gap at the bottom: critters can get in with no problem at all. Therefore the addition of the bricks, to close that gap. Need enough for the width of the door, about 30", and someone had suggested regular bricks. I haven't measured yet, but those will probably work.

What do I do? Open the bucket, wet the slab, slap some mix onto it, press the bricks in and cement them together? I suppose I need to be careful to be sure any slope is down away from the door, right?

What else?

I may then install a rubber what's that called, along the bottom of that steel door.