It really is as simple as scanning the horizon for a crane, travelling to the location, scouting it out to see the best way to gain entrance unseen, then (I recommend) coming back a couple of days later and just going for it. You can go for it same day but I like to scope a place's perimeter a few times before I send it just so I can ensure I know where cameras are / identify any potential patterns in terms of security presence etc. You'll have a decision to make when you're at the fence, let your excitement turn into fight instead of flight and climb! Once you're over the fence just execute your plan and don't deviate unless you're spotted.
If the crane is locked (they generally are thanks to Ally Law and James Kingston and dickheads like that) then if all you want to do is climb and be high up, most construction sites have these matrix pillar structures on the outside that you can climb. I've climbed a few of them when I've been disappointed by a locked crane. But they're harder / more dangerous and if you don't have the right leg length you'll find it hard. Pic below is example cus I didn't really explain it well. The thing on the right, you can't miss them.