ok... there are 2 ways of activating docking squares for the planets/bases - by pressing d (as mentioned by keller) or pressing alt+0 (it's more convenient cos when u are in range for docking with a planet and you request to dock with a base, you dock into the planet instead.)
as for slow speed while docking...
1) press y to switch to travel mode. it will multiply ur speed by 1,000 times.
2) if u are willing to do some basic repairs and have bought overdrives, press tab to afterburn and increase the rate of increase for velocity.
by doing the above 2 steps mentioned, you can dock faster than before - use only when confronted with a planet. when u try to do this at a base:
1) most probably crash into the base.
2) and even more probable, overshoot the docking square and you have to make 180 degrees turn.
hope this helps~
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