How am I supposed to understand 充分一些？
First: 充分
I feel like the definition ample is slightly misleading, although it could be translated this way in specific contexts.
A Chinese-English Dictionary:

2 to the full; as fully as possible
充分利用
fully utilize; make full use of

Oxford actually translates, what most translate as ample, as abundant.

Second: 一些
Oxford

3 略微 [placed after an adjective, indicating a slight change in degree]
请走慢一些
Please walk more slowly

So, we can think of an 一些 suffix as &quot;a little more....blah blah blah&quot;, it is adding to whatever is already there.
一些 = and some

Now we have 充分 + 一些 = a little more abundant (-or- a little more ample, if you really want to use ample) -or- with 准备 in mind: to prepare a little more fully

Now 尽量
KEY:

to the best of one's ability; as far as possible


尽量准备充分一些 = you should prepare a whole lot (more), as much as you possibly can