Whilst I haven't been for some time, I have been there on a number of occasions.
As has already been described, Cerhenice is the station you want, not Velim. That said, next to Velim station is the exchange siding, which sometimes holds stock on transfer to/from the test track. If you fancy doing most of the journey there loco-hauled from Prague, then simply double back at Kolin, which isn't far away.
From Cerhenice station, once you reach the road junction at the top of the village, you can go virtually straight on, which will take you to the test centre's entrance, then walk to the left to see the stabling sidings from the west side. Turning right at the road junction at the top of the village (i.e. following the blue dotted line on the map poted earlier) takes you to the bridge over the outer circuit. Be aware that there is no footpath along this road. It isn't particulaly busy, but is used by lorries, etc. It is possible to observe any movements on the outer circuit from the road bridge.
The route with the red dotted line on the previously posted map is feasible, but may be muddy. You can see lots stabled stock from there. Where the red dotted line turns right at the top of the map, this takes you directly to the inner circuit, where you will be able to see anything testing on there. Trains can be tested on both the inner and outer circuits at the same time.
There are signs around the outer circuit saying no photography, but I've never had any issues.
As my visits were between 2017 and 2020, some of my info may be out of date. This was a fruitful period for UK trains on site - I photographed British 68+Mk5's, 195, 196, 331, 397, 701, 745, 755 and Mk5 sleepers alongside stock from around Europe. I've also been lucky enough to travel around both circuits on a charter train, back in 2014, using a diesel "Grumpy" (751010) and electric 124601 from their home fleet.
I recommend mapy.cz for their lovely maps:
https://en.mapy.cz/zakladni?l=0&x=15.0912539&y=50.0954293&z=14
Here's a link to the Youtube site of a local, who spends a lot of time there:
https://www.youtube.com/c/TomášPelech
and this is his flickr site:
https://www.flickr.com/photos/144778013@N06/