Looks like a nice jumper. The first thing I would do is print the pattern which can be done by clicking on the Pattern button. Then I would work out which size I was doing, and highlight the correct number for that size Size: S - M - L - XL - XXL - XXXL so size XL will be the 4th figure. Apologies if you have already got this far. The rest seems fairly conventional
It looks like it will become easier as you get used to the pattern. I'm pretty much bilingual with crochet patterns these days. The charts are useful. I would work slowly, taking care to follow the chart where advised to by the pattern. Try making gauge swatch first before starting on the garment, to get used to the pattern terminology. It looks pretty easy. If I can find some spare yarn I will do a test sample and may be able to advise you later. I'll get back to you this afternoon.
Right then, from the starting chain onwards: As I understand it you join starting chain to form a circle, be careful not to twist the chain. (I always find this tricky). Round 1: Work 3 chain (this counts as your first treble) then work one treble into the next five chain, miss a chain then one treble into each of the next six chain, continue until the round is complete and join with a slip stitch into the top of first chain 3. Round 2: Chain 1, double crochet in next stitch, chain 1 miss next stitch double crochet in next stitch all round, join with a slip stitch in first chain 1. Round 3: Chain 1 make 3 trebles in first one chain space, double crochet into next double crochet all the way round, join with a slip stitch in first chain 1. Then follow the diagram A2. Here's a photo of my sample, sorry it's a lurid pink but I hope it gives you some idea.