First, coat the lips of the seal and the outside of it with a smear of grease.
I guess you aren't using a seal drift so you need to use a small drift, about 10mm diameter and gently tap the seal into the leg a bit a time. The more you keep it level, the easier it will go in. It'll go in quickly at first and this is where you need to take extra care to keep it level.
Once it's in about 50%, then it'll keep itself staright and you can concentarte on getting it fully home and seated in the fork leg.
Patience.........................