The first is to cross the border of Poland only in specially designated places. Uncontrolled violation of the border will cause serious problems on the part of Ukraine and Poland, up to a ban on visiting the last country for six months or more.
Second, you must have documents with you:
These are mandatory documents that will be 100% verified. There is also a list of additional papers that should also be with you:
Be prepared for questions: at the customs control and verification of documents you may be asked:
And other questions. Do not get lost, answer honestly and there will be no problems.