You need some hot glue, 3mm x100mm rods, 4mm x 100mm rods and some landing gear tires as well as soldering your own three-way Y connector little landing gear.You're also going to need some sort of Dremel or cutting tool that can cut through 3mm steel rods.
The landing gear servos will need to be screwed into the fairings with the included for screws from the bottom side (If you glue the fairings to the aircraft before installing the servos, you can't install the servos). You'll need to hot glue the main landing gear fairings and the nose landing gear fairing as seen in the pictures. Then you can install the 4mm by 100mm rods into the landing gear servos.
Update May 27th 2018
The length of the nose landing gear and main landing gear rods are now 100mm in length and 4mm in diameter. To install the wheels you will need the 3mm diameter rods cut to length and for the steering servo in the NLG. A paperclip can be used for the speed brake hinge. I have changed the 3D printed Parts accordingly to fit the new lengths.
Sutemribor 3mm x 100mm Model Straight Metal Round Shaft Rod Bars for DIY RC Car, RC Helicopter Airplane (10 PCS)
Uxcell Round Rod (10 Piece), 100mm x 4mm
Goodlife623 Servoless Retract Landing Gear with Metal Trunnion-DIY Retractable Gear
Akfix 705 Fast Adhesive CA Glue (0.7 oz.) with Activator (3.38 fl oz.) [Clear Super Glue Adhesive and Accelerator Spray]
Du-Bro 140 5/32" Dura-Collar (4-Pack)
NLG wheel x1
175SL - 1.75" (44.45 mm) Axle dia 1/8" (3 mm)
4.4 grams/wheel width 0.6875" (17.5 mm)
Main landing gear wheel x 2
250SL - Diameter 2.50" (63.5 mm) axle dia: 5/32" (4 mm)
11.5grams/wheel width: 0.7813" (19.8 mm)