Insert from the rear. Straighten the 2 tabs at the front of the liner with a toothless pliers before inserting. Leave the rear tabs alone. The trick is to keep the rear tabs fully seated against the wood as you bend the front tabs back against the face of the front handguard ferrule. When the front tabs are fully inserted bend them back against the face of the ferrule with a brass punch.
I have made up a fixture which let me keep the rear tabs fully seated as I bend the front tabs back into position. I also use a tapered plug to let me bend the front tabs outwards before seating them fully. It is also useful to use a wooden or metal block that fits the inside of the liner tightly to avoid bending the liner inwards while you are doing all of this. The front tabs are a caution as the metal can work harden and break as a result of too much bending/straightening. One could try annealing them with a torch to soften them.
Be careful of some of the Boyds handguards as they are either too shallow or too deep for the rear tabs to seat properly in order for the front tabs to project enough beyond the front ferrule to bend them properly. The front of the handguard liner channel portion should seat far enough to contact or just about contact the rear face of the front ferrule. You might need to relieve of fill some of the Boyds guards enough to achieve this fit.