It will be better to do USMLE step 1 after finishing the 3rd course and not to delay it for later in order not to forget all MBBS subjects that you will not exercise so much in the future after 4th course, I mean during the clinical part.
for USMLE step1 preparation, you may use Uworld QBank, FA textbook, Kaplan lecture notes, Lecturio medical videos and MCQs, amboss QBank, BnB videos
step 2 can be taken after the 5th course, so until this time you should be already gone through the all-important subject that you need for USMLE step 2 ck.
for USMLE step 2 CK, you can use Uworld Qbank, step up to medicine textbook, Lecturio videos with QBank, Onlinemeded, and medbullet.
Ahmad Ismail, M.S from Kharkov National Medical University (2019)
Source: Quora Digest