Hi! I'm about to buy my first pair of skates. I cant go to any physical shop, they are all several hours away from where i live, but decathlon is selling Risport Venus this year and i'm getting those. But because it's decathlon i doubt there will be professionals and i'll have to do the fitting myself. I've read your guide to buyins skates but do you have any specific tips as to what i should look for? (as in, should i or not be able to fit my finger behind my ankle, etc). Thank you so much!
I found a great comprehensive guide for trying skates here
Here is the bit about trying them on
Follow these steps for trying on skates:Â
1. Unlace the boot very loosely and pull the tongue forward as far as the laces go.Â
2. While seated, insert the foot. Slide it all the way forward, being sure to keep toes flat.Â
3. Check to see if you can insert your index finger behind the heel. Ideally, it’s a tightsqueeze – only half a finger width is recommended. If you can get a full finger in, the bootis too long. If you can’t fit it in at all, the boot may be too small.Â
4. Now kick the heel to the very back of the boot. Curl the tongue in around the foot andankle and do up the laces firmly, but not too tightly. If there is extra lace, do NOT wrap itaround the ankle. You can go back down and run it up the hooks of the skate one more time,and/or double knot it at the top.Â
5. If the sides of the skate are stretched apart so they don’t cover the tongue, the bootis too narrow/small. If the sides of the skate appear too close together (almost meetingacross the tongue), or if the skate appears to buckle or bubble near the toe seam, theboot is too wide/long.Â
 6. After lacing up both skates, stand up in them. Toes of both feet should be laying flatand NOT touching the ends of either skate.Â
7. Take a walk in the skates. Heels of both feet should NOT lift at all, and should besnugly in the skate. This is the most vital of fitting steps!!!! The heel should always staysnugly in the heel cup of the skate.Â
8. The skater should be able to stand up completely straight, without any leaning in or outaround the ankle area. If the ankles are buckling while standing, the skates do NOT haveenough support.Â
9. If the skater can walk comfortably, supported through the ankle area, with the heelssnugly in the heel cups and no toes touching the fronts of the skates, you have found agood fit!!!Â
‘RISPORT LENGTH IS MEASURED IN MONDOPOINTTAKE YOUR FOOT MEASUREMENT IN CENTIMETERS (CM)THEN ADD 1 CM TO YOUR MEASUREMENT TO GET YOUR BOOT SIZE.’ (from here)
that is the Risport sizing chart.
If you are still growing you want half a size or so up, meaning fitting one finger comfortably down the back (or two at the point where kids go up a size every few months). If you are not growing In my skates I can fit one finger down the back but it is a tight fit, my nails just touch the front of the boot (exact size of my feet, the leather will stretch a little as the boot breaks in)
The fit should not cause you significant pain.
That PDF covers it comprehensively :)