Squiggles and Twiddles

Here's a question from one of my readers which perhaps others may find helpful.

The question is - What's up with the pluses and squiggles?😅 (Boismortier Op. 8)

The signs suggest different types of trills. The pluses tend to happen on longer note values and the squiggly lines on shorter ones. So I would interpret the squiggly lines as a shorter, quicker trill (4 notes), very superficial and played absolutely lightly (using hardly any air at all, in diminuendo). It should just sound like something fluttering in the air. For the pluses, there will be more room for a slightly longer appoggiatura (but with varying lengths depending on the instance), starting perhaps a bit slower and then getting faster as time permits, and overall a longer trill. Lentement does mean slowly, but it is still indicated in 4 (the C) so we want to feel 4 big beats.
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If you're having trouble with this rhythmically, try to see if you can feel both the big pulse as well as the 8th at the same time. The longer trills will benefit from that inner subdivision.