1. Use surface retarders. These admixtures are sprayed on the finished surface and penetrate only the very top layer of cement. They will prevent this layer from hardening. A few hours later - after the rest of the concrete hardened enough - you can come back with a broom and/or hose to expose the aggregates.

2. Abrasive materials. Use steel brushes (or similar) to remove the cement cover before it hardens too much, but after it gained enough strength so that the work will not dislodge the aggregates. Choosing the right time can be tricky.

3. Acid wash. Use muriatic or phosphoric acid to dissolve the cement on the surface. Use diluted acid and practice on scrapes. This can be done after a few days. Acid wash may also be used to finish concrete done in one of the previous methods. Be careful not to overdo it and damage the deeper concrete and/or the aggregates.

4. Water blast. Get high pressure water blasting equipment. Use it after the concrete is strong enough (12 to 24 hours) so the water does not blast the aggregate out. Depending on the type of concrete and temperature, you may have to wait a few days.

Note: If you want to use fancy (expensive) aggregate, it is not necessary to use it for everything. Most contractors will use regular concrete, and spray the finish aggregate on the surface. They will tap it into the concrete surface to achieve a flat finish, and use one of the above methods to expose it later.

Be aware that if you never did that before it is possible (likely) that your first try will be less than perfect. Get professional help if you can

Good luck