Did you look at a map? Sequence makes little sense... Sifnos is the outlier, hard to make connections because it's in the WESTRN Cyclades. The most time-efficient sequence would be:
• FLY to CHANIA (Western Crete)... devote at least 5 days to Crete ending in Heraklion
• 9:30 AM ferry to SANTORINI - limit to 48 hours, for the"famous View" - try to time it for F-S-S, Weekdays are cruise-ship Hell
• daily ferries to FOLEGANDROS -- beautiful CHORA where u want to stay, 3 days is plenty...
• In July - daily 11:30 ferry to NAXOS - convenient but small ferry, bouncy ride; W & Su a large stable ferry 12:30
• Many daily ferries Naxos- PAROS (1 hr)
• MYKONOS??? VERY overpriced/crowded in High season. Consider a day-excursion from PAROS that takes you to both Mykonos AND Delos & gets you back to PAROS to enjoy the sunset.
• from PAROS, the BEST & Fastest & most STABLE daily ferry is 18:25 -19;15... a car-carrying catamaran, heavier more stable.
• once on SIFNOS, many options for return to mainland.
You won't have to buy tickets for any of these online before y our trip -- when u get to an island just buy tickets at an agency for the next "hop". Only one that MAY be in demand in High season is Heraklion - Santorini... only 2 per day, high demand. If you buy when you arrive in Crete, for 5 days hence, should be fine.
You can do it on your own IF willing to put in time & effort. In High Season, on a budget, you are going to have to strategize for lodgings --- Go to booking.com for each destination, plug in your desired dates (i.e. CHANIA July 15 - 18). a list of places will pop up. But look in upper RH corner & click on a box labeled "map view". All places available thru booking will show on a town map as Blue Balloons. Red centers mean fully booked for your dates; white centers still have rooms. when you touch a balloon, you see room rate. In July, you don't have luxury of waiting til you get to a place... you MUST book ahead. Yes, you CAN do AirBnB & can proably get better rates... but Booking.com will give you an "educ ation" as to what the going rates are, and what the good locations are. Otherwise you're a babe in the woods. GOOD LUCK!
NOTE: If you are limited in travel experience and plan a MONTH in Greece I hope to Hxll you are investing in a good Guide Book - NOW. I cannot imagine how someone can plan to spend thousands and thousands on a major trip, and still not invest $20 for a reliable guide book. You cannot do a D-I-Y without experience just using a smart-phone! For you, probably lonely Planet Greek Isles is the best most compact one. Buy it today.