A Perfect Reaction (see all): You find a procedure that worked on a system very similar to yours, and all the functional groups in your molecule will be compatible with the reaction conditions. The procedure is described in detail by a respected research group, and has been referenced often by others.
A Nightmare Scenario (see all): Theory suggests that the experiment will work. However, the only precedent you find is either (a) in another language (b) from 1927 (c) has no experimental explanation or (d) was published by a group nobody knows about and has never been cited.
Step by step:
Do a database search to see if the reaction has been done before
on a system like yours. Notice how many different authors report similar chemistry.
Look at some of the papers describing the transformation and take note of
how many examples have been reported
how general the chemistry seems
how easy the procedure is to carry out
whether the reagents are available and inexpensive
how detailed the experimental description is
the authors of the paper and the journal it came from
If possible, find a review article describing the reaction and read about the scope and limitations of the chemistry
Always find and read the original paper that popularized the transformation
Choose a procedure to follow, taking all factors into consideration
Copy or print the entire journal article, not just the pages you need.