It also depends on if you are using PDMLink or ProjectLink:
Option 1 of Shreyas's response is available in both.
Option 2 is available in PDMLink only.
For ProjectLink you can also use the "route" action on an object, which has I believe 5 default configured workflows that you can set up approvers and/or reviewers before kicking it off, as an ad-hoc approval process.
The option you choose also depends on the ability to search for the approval processes and record of approval as well, so make sure to keep that in mind.