As an undergraduate, you should generally apply the summer before your senior year starts. That is, of course, if you want to matriculate into a DPT program right after undergrad. Make sure that you have:
-Prerequisite requirements for each program
-GRE completed/Planned for completion
-Observation Hours
-Letters of Recommendation
-Work on your resume(for some programs)
-Personal Statement
-Transcript(s) from ALL schools you attended
The earlier you can have all of this stuff done, the better you are. PTCAS typically opens July 1st for e-submission of your application. Then it takes approx 3-6 weeks for them to mail it out to your schools once they have matched all of your information with your transcripts. My advice would be to go over your application several times before you submit it. What may seem very trivial or marginal to you could cause a delay in the processing of your application. Once you do that, you will be good to go. I am sure there will be questions along the way, but this is a general guideline. Also keep in mind that depending on the schools you choose to apply to, there may be supplemental applications/materials to fill out. Best wishes to you!