This is a 4mm / 1:76.2 model of a L&YR Tender chassis in EM.
The chassis is essentially complete, all you need are 3 sets of 14mm diameter wheels and if you choose 6x 2mm brass 'straw hat' bearing cups. Test builds indicate the wheels roll nicely with or without bearing cups. You may need to ream the holes out for the axles a little but If you are using the bearings then the holes will definetly need reaming out.
Fit one wheel to the end of a flush axle and thread a spacing washer or two on, then thread it through the bearing carrier (or bearing), then add a spacing washer or two then the wheel to the other side and gauge it with a back to back gauge, do this to all 3.
The chassis fits to the body by slotting the front into the body at an angle and raising the rear until level, the hole in the chassis will line up with the hole in the body. Once done it is fitted with a screw or bolt, the model materials allow the screw or bolt to self tap and a retaining nut is not needed even after repeated removal and refitting, however space is reachable on the inside to add a nut if you prefer.
As an alternative you can cut the bearing carriers off completely but retain the brake gear, then use Pinpoint bearings instead for the wheels and seat the pin points into the indented cups on the inside of the frame, just lightly spring the model to fit them.
As a 3rd alternative if you are not using the optional chassis at all then you can use Pinpoint bearings as above but If using this method you will have to obtain/make your own brake gear.
Add the buffers, your choice of couplings and tender connector and you're ready to roll.
For wheels, axles, crankpins etc
Alan Gibson is a great resource
...as is Markits
For Motors and Gearboxes, High Level are recommended.
For crank pins, hand rail rod/wire and pillars, washers, bearings and other small sundry items Mainly Trains is a good website to visit.
But of course you can use whatever suits your needs and preference best,
the above has been typed out to hopefully aid you should you be new to kit building. The advantage of these 3D printed kits is 90% of the work is done for you.