I have a method which is different from most, which I think has advantages.
First it needs to be stressed, what is the purpose?
These leading-in-to-the-work-set sets can have intended purpose of:
A) Having training effect in their own right plus ensuring being able to work well with the work weight.
The cost to this is doing this consumes, so to speak, some of the training volume that is possible for you to use that day. It's not unusual for a lot of the volume that a given lifter might do to get taken up this way.
This can be fine, as the training benefit may exceed what would be the case if the volume had been employed another way, but it should be recognized that seeking training effect in these working-up sets does have a cost as well.
B) Having little to no training effect of their own right, but existing solely to perform best and safely in the real work set or sets.
Where my goal is the second of those, my method is:
1) Extremely light warmup with for example about 15% 1RM. This might be 30 reps, or 3 sets of 10, or 3 sets of 5, or whatever seems best to me. This is to get blood flowing and some basic warming with no real drain on the system.
2) A method I took from Doug Hepburn's methods: a set of 5 at 50% 1RM, then a single at 60%, 70%, and etc if need be, until the weight is within one increment of the work weight. E.g. if the work set will be 85% 1RM, I would do the singles up to 80%.
I expect there is very little to no training effect from this method. But it also very little reduces the total volume of real work that one can do in the workout.
Sometimes it is simplified from this. For example for EZ--bar curls I will do three very fast sets of 10 using the lightest bar with only 15 seconds rest between them, and then use a Hepburn-like approach with singles but at 10 lb intervals.
It doesn't have to be exact.