The SOP is your friend. In COM, it really helps keep things straight. To your answer above
"put a siege marker down correct?"
"Wait until all activations done?"
You can only put a siege marker down normally at the end of all activations (iic below) (spending an SA), and if the space meets siege criteria. Or, sometimes, if your force STARTS an activation already on an enemy fortress and meets siege criteria (i below). Then you could expend an SA right then to set the siege.
Siege rolls - the most you can get is 2 a turn. One normally during the siege segement, for free (3 below). IF your force is besieging AND it has been activated this turn, you can spend an SA to get an early siege roll during the Activation (c below) (you'd still get the free one in the siege segment.)
SOP, strategy segment:
Finish each Command’s activation before beginning another’s
(Exceptions: Combined Offensive, Converging Attack [8.6.2])
by completing this sequence in order:
i. Set Siege—If the activated Command meets siege criteria
when first activated [17.4], it may expend an SA and place
a Siege 0 marker.
ii. Movement—Move the activated Command.
iii. Combat—If the activated Command is in an enemy-occupied
space, it may attempt to initiate battle, or an inactive enemy
force may attempt to initiate battle against the active Command.
c. After all activations are complete, may expend 1 SA for each enemy
fortress space with one or more activated Commands that did not
fight a battle this Turn and now meet siege criteria to place a Siege
0 marker or (if a Siege marker is already present) make a roll on
the Siege Table. Note: N/A in Severe Weather [6.4].
d. May activate one Minor Force that has not already activated or
conducted an Admin March this Turn for an Admin March.
(3) Siege Segment—Make 1 roll on the Siege Table for each fortress
space with a besieging Command controlled by the active Power
Glad you're getting into it! For sure, this is not a streamers and training wheels tricycle CDG. I will submit this is the best strategic simulation of the SYW out there, in CDG format. It is meaty, but it will provide a high brain investment/historical drama return ratio if you make the commitment.