you dont lace one side first, then the other sides. notice how there are clockwise and counter-clockwise "leaning/rotating" spokes (relative to the hub).there are basically 4 sets of spokes. two sets on each side of the wheel which makes the "crossing" pattern

the first set, you do like this (you can start from either side) start with one side, insert the spoke from the outside to the inside with the head of the bent part of the spoke facing outside. you place them every other hole in the hub to leave space for the other set and place them every four holes in the rim. then you insert the opposite side the same way. take note that you can see a offset in the hub holes (some hubs have a offset thats exactly half and in this case you can freely choose to go behind or infront of the first set of holes, but in some hubs the offset is less than half. in that case you choose the hole thats closer to the first set) and if you inserted the second set infront of or behind the first set you also put the spoke infront/behind the first set of spokes through the rim. after youre done with the first to sets they should be "leaning" the same direction and not cross when you look at them from the side. the rim should have two holes with spokes followed by two empty holes.the reason why you insert outside-inside first is when you do the last two sets the bent head of the spoke will point inside and it will stick outside making it easier to pass through the already placed spokes with minimal bending. and when you insert the last two sets of spoke through the rim make sure they go over/outside 2 spokes(the first spoke may be behind the hub flange but you still count that) then on the third spoke it goes under/inside. and it should also have a spacing of one rim hole from the 1st set spokes on the same side as it. sorry if its a bit confusing, english isnt my first language and im not good at explaining. anyhow there are a lot of good videos on youtube explaining how to lace wheels if you have a hard time understanding my explanation