Measure random fast signal

[cmug]

I use for years an CTR scope for audio measurements. Last year I switched to an Pico Scope, and that's fine.
Always measured with continuous signals and tone generator. Now I do other experiment with Geiger counter. Bought an high voltage unit with TTL output. Now I want to measure the output and see if the square wave is OK. But I have no idea how to trigger this. It’s an random fast signal. You can see what I mean in this video
https://youtu.be/efs6gcmu8zs?t=40s
I have an Pico scope 2304 and use Mac OSX but I think question is the same on PC. Can help anyone with tips, explation (website?) how to measure this. The signal is to fast to start and stop by hand.
From The Netherlands: Clemens

[Martyn]

You should be OK by setting a trigger in the bottom toolbar. Initially set this to Auto so that the software will draw a trace every second if no trigger is seen. Then drag the yellow trigger diamond on the screen to get a sensible trigger. Finally change to Repeat trigger so that you only get a trace when the trigger fires.

If you are still having difficulties post a psdata file to show the signal and your setup.

[cmug]

Thanks.
I thought too much. Now I used start an just done it. Connected and few mouse clicks and it worked!
Never thought it was so easy.