You can talk to your minister to send an envoy, who's gonna declare a political war on your enemy.
From the inofficial wiki
"By talking to this minister you can conduct many different affairs of
state. Diplomats can be dispatched to the other factions, performing
such actions as declaring war..."
Sadly, it would still cause problem with your honor rating and lord relations. It only prevents you from losing relationship with city/village/lord you wolud have to harras otherwise.
In order to declare war without consequence, you have to wait for event informing about border incident.
In game, you can consult with all your lords before appointing a fief, then appoint the fief to the lord who has the most supporters amongst other lords. This should limit the negative relations hit to the lords who did not support the lord who eventually gets the fief.
Failing that you can modify your game using the module system to lessen the relationships hit. IF you're not familiar with modding the game and the module system this can be a bit of a challenge, but the way to do it is:
Find the file module_scripts.py, and look for the script give_center_to_lord.
Find the line:
(val_clamp, ":relation_with_liege_change", -5, 3)
This line signifies that the relationship change between lords and liege when awarding fiefs varies between -5 and 3. Change those values to whatever you like, for example -1, 20.
Note that this change will also affect other kings, so this makes getting their vassals harder.
To make the change only affect the player, change the line by this piece of code:
(try_begin),
(eq, ":faction_leader", "trp_player"),
(val_clamp, ":relation_with_liege_change", -5, 3), ##CHANGE THIS LINE HOWEVER YOU'D LIKE
(else_try),
(val_clamp, ":relation_with_liege_change", -5, 3), #THIS WILL BE FOR ALL OTHER KINGDOMS
(try_end)
Again, change the values in the above piece of code to whatever you want.
Source: talewords forum.
Best Answer
The easiest way to do this with vanilla Warband is to ride to the location you want your troops at and order them to "Hold this position" with F1 after selecting the kind of troops you want there ( 1 -> F1 would order everyone there ).
If you want to use those Flags like the players in the videos you have seen, you probably should use Floris Mod to play the game, since this mod adds Regiment Flags.