I'd take about 6' of small line and double it in half. Then pass the 2 ends through the shackle or loop on your block and then through the loop in your line to form a cow hitch with two equal legs of line.
Place the block on your pulpit and ( in opposite directions) wind
each leg around the pulpits trough the shackle loop a few times. The actual number of turns will depend on the strength of your line and will be a judgement call.
Once you get your turns, cross your lines and wrap them in opposite directions around your turns between your pulpit and the block, pulling your lashing tight. Make a couple turns that way and finish with a reef knot.
Of course there is a good chance that, like I sometimes do, you will discover you needed a longer piece of line about halfway through the process but that is part of the learning
I hope this helps. Preventer lines can take a lot of strain so I'd be sure to check your lashings to make sure they are in good shape. Also, it is possible for them to slide on stainless tubing depending on where they are placed. Of course, you could also bend your pulpit! Have you considered putting the blocks on your toe rail? The lower angle might be helpful and they are much stronger.
Best of luck!