When I was a newbie I slowly dipped my toes in the water.... I started out with one shop a day or every other day. When I felt comfortable, I did two shops and then worked my up to 3 shops a day. ....
My best advice.
1. Go slow. Take only what you can handle and work up. Never take a slew of unfamiliar shops. Take one and then if you don't like that shop, get it done and put it behind you. Don't take that shop again.
2. Never cancel a shop unless you have extreme circumstance. Canceling because you don't like the amount of work in the guidelines, to me is wrong. (everyone has their own opinion on this one).
3. Higher paying shops usually have quite a bit of work. I have not done the Swedish Furniture Shop but others who have said for $60-90 dollars it's a LOT of work. As you get more experience, you will realize higher paying shops require more work. Even some lower paying ones have tons of narrative and observations you need to capture.
4. The most valuable advice I can give you: What you do as a shopper reflects on your record with that individual Mystery Shopping Company........(hypotheticals)--> If you overwhelm yourself with work, turn in reports that don't follow the guidelines, then you don't get paid, they report this on your file, and they can share grievances with other schdulers so your mishaps can spread throughout the industry.
Go Slow. Get it Right. Turn in Great Work, otherwise...what is the point of mystery shopping if you are getting (hypotheticals) callbacks, inquiries, missing receipts, failure to follow guidelines, etc. It "does" matter what you do as a shopper, every single time.