i would stick with MGMAT SC for sentence completion... read it more than once (you will pick new things up each time you review it). also, make sure you carefully read the solutions in OG
for CR, i would use powerscore (critical reasoning bible). the trick with CR is full understanding the different types of questions stems (e.g., weaken, strengthen, assumption, etc.) and how best to attack them. there is a different strategy for each type of stem.
i would decompose studying into 6 categories:
(1) subject material/mastering the basics
(2) learning the format of the GMAT inside and out. learn basic POE patterns on both quant and verbal
(3) working on timing / accuracy (work under time constraints while working through practice/OG problems . i would typically do 10-20 problems at a time under GMAT timing (e.g., 2min for quant and 1min 45sec for verbal) and then check my answers. it's important to save OG problems for AFTER you master material. you may get problems you have already seen and answer them correctly (from memory) and never address that weak spot
(4) building stamina (by taking ~1 CAT per week). common mistakes are (1) taking too many CATs and not using that time for studying the basics and (2) not working under test conditions (AW, quant, and Verbal, with 8 min breaks)
(5) study your mistakes. figure out where you are consistently making mistakes and study the material more
(6) strategy
i put them in order of importance (at least in my opinion).
remember, taking CATs before you master material is a waste of time. that time could have been spent learning and mastering material. time you lose. moreover, don't focus on your test scores. rather, use the results to review mistakes (look for patterns). the GMAT PREP will be the best predictor of you actual GMAT exam, but keep in mind that your scores will be inflated when you retake them.
if i was you, I would spend a solid 4 weeks working on the basics. then spend 4 weeks doing problems and taking 1 CAT each week. take the exam the first week of June. your score will improve. it's going to take a lot of time and energy, but it will be worth it. use your time wisely! practice CATs show you where you're making mistakes. they dont improve your understanding of the material.
good luck!
